Support host:guest mount syntax with WORKINGDIR resolution

--mount now accepts <host-path>:<guest-path> (Docker-style). When the
guest path is omitted, the host path is used. Non-absolute guest paths
are resolved against the image WORKINGDIR, which is written to
/slim/workdir at build time and read by slim-init.sh at boot.

Changes:
- qemu.rs: Parse host:guest spec, pass guest path (not host path) on
  the kernel cmdline as slim.mount=<tag>:<base64(guest_path)>
- inject.rs: Accept working_dir param, write /slim/workdir into rootfs
- build.rs: Pass config.working_dir to inject()
- slim-init.sh: Read /slim/workdir, resolve relative guest paths
  against it before mounting
- test.sh: Test host:guest absolute paths (multi-mount) and relative
  guest path resolved against WORKINGDIR

Addresses PR #9 review comment from @hulthe.

example/test.sh

@@ -107,9 +107,13 @@ test_mount() {
     echo "=== Testing --mount (9p shares) ==="
 
     work="$(mktemp -d)"
+
+    # --- Image 1: no WORKINGDIR, test host:guest and multi-mount ---
     img="slim-test-mount"
     share_dir="$work/share"
     share_dir2="$work/share2"
+    guest_dir="/mnt/guest"
+    guest_dir2="/mnt/guest2"
 
     mkdir -p "$share_dir" "$share_dir2"
     echo "hello from host" > "$share_dir/testfile.txt"
@@ -120,7 +124,7 @@ FROM alpine:latest
 CMD ["/bin/sh", "-c", "poweroff -f"]
 EOF
 
-    echo "-- Building container image..."
+    echo "-- Building container image (no WORKINGDIR)..."
     if ! podman build --network=none -t "$img" -f "$work/Containerfile" >/dev/null 2>&1; then
         report fail "--mount (podman build failed)"
         rm -rf "$work"
@@ -135,25 +139,63 @@ EOF
         return
     fi
 
-    # The 9p share appears at the same path inside the VM as on the host.
-    # Use --cmd to run the verification snippet with the canonical path.
     share_canon=$(readlink -f "$share_dir")
     share_canon2=$(readlink -f "$share_dir2")
 
-    echo "-- Booting VM with --mount $share_dir --mount $share_dir2 ..."
+    echo "-- Test 1: --mount host:guest (absolute guest path, multi-mount)..."
     output=$(timeout "$TIMEOUT" "$SLIM_BIN" run "$img" \
-        --mount "$share_dir" \
-        --mount "$share_dir2" \
-        --cmd "echo Available 9p shares:; for f in /sys/bus/virtio/drivers/9pnet_virtio/virtio*/mount_tag; do [ -f \"\$f\" ] && echo \"\$f: \$(tr -d '\\0' < \"\$f\")\"; done; echo Mounts:; mount -v; cat \"${share_canon}/testfile.txt\" 2>/dev/null || echo NO_FILE; cat \"${share_canon2}/testfile2.txt\" 2>/dev/null || echo NO_FILE2; echo MOUNT_VERIFY_DONE; poweroff -f" \
+        --mount "${share_dir}:${guest_dir}" \
+        --mount "${share_dir2}:${guest_dir2}" \
+        --cmd "for f in /sys/bus/virtio/drivers/9pnet_virtio/virtio*/mount_tag; do [ -f \"\$f\" ] && echo \"\$f: \$(tr -d '\\0' < \"\$f\")\"; done; mount -v; cat ${guest_dir}/testfile.txt 2>/dev/null || echo NO_FILE; cat ${guest_dir2}/testfile2.txt 2>/dev/null || echo NO_FILE2; echo MOUNT_VERIFY_DONE; poweroff -f" \
         2>&1 || true)
 
     check_output "$output" "mount_tag: slim0" "--mount 9p share detected"
     check_output "$output" "mount_tag: slim1" "--mount second 9p share detected"
     check_output "$output" "type 9p" "--mount 9p filesystem in mount list"
-    check_output "$output" "hello from host" "--mount file accessible in VM"
-    check_output "$output" "second share" "--mount second share file accessible in VM"
+    check_output "$output" "hello from host" "--mount file accessible at guest path"
+    check_output "$output" "second share" "--mount second file accessible at guest path"
     check_output "$output" "MOUNT_VERIFY_DONE" "--mount VM ran to completion"
 
+    cleanup "$img" "$img"
+
+    # --- Image 2: with WORKINGDIR, test relative guest path ---
+    img="slim-test-mount-wd"
+    share_dir3="$work/share3"
+
+    mkdir -p "$share_dir3"
+    echo "relative share" > "$share_dir3/relfile.txt"
+
+    cat > "$work/Containerfile.wd" <<'EOF'
+FROM alpine:latest
+WORKDIR /app
+CMD ["/bin/sh", "-c", "poweroff -f"]
+EOF
+
+    echo "-- Building container image (WORKINGDIR /app)..."
+    if ! podman build --network=none -t "$img" -f "$work/Containerfile.wd" >/dev/null 2>&1; then
+        report fail "--mount relative (podman build failed)"
+        rm -rf "$work"
+        return
+    fi
+
+    echo "-- Building slim VM..."
+    if ! "$SLIM_BIN" build qcow2 "$img" >/dev/null 2>&1; then
+        report fail "--mount relative (slim build failed)"
+        cleanup "$img" "$img"
+        rm -rf "$work"
+        return
+    fi
+
+    echo "-- Test 2: --mount host:relative-guest (resolved against WORKINGDIR)..."
+    output=$(timeout "$TIMEOUT" "$SLIM_BIN" run "$img" \
+        --mount "${share_dir3}:data" \
+        --cmd "mount -v; cat /app/data/relfile.txt 2>/dev/null || echo NO_REL_FILE; echo REL_VERIFY_DONE; poweroff -f" \
+        2>&1 || true)
+
+    check_output "$output" "type 9p" "--mount relative 9p filesystem in mount list"
+    check_output "$output" "relative share" "--mount relative file accessible at WORKINGDIR/data"
+    check_output "$output" "REL_VERIFY_DONE" "--mount relative VM ran to completion"
+
     cleanup "$img" "$img"
     rm -rf "$work"
 }