Add --mount flag for 9p host directory sharing

Add support for sharing host directories into the VM via QEMU 9p
(virtio-9p).

The --mount flag can be repeated.

The format is `<host-path>` or `<host-path>:<guest-path>`

Relative guest paths are relative to the configured WORKDIR.

Design:
- QEMU: each --mount gets a short 9p tag (slim0, slim1, …) via
  -virtfs local,path=…,mount_tag=slimN,security_model=mapped-xattr.
  Tags are kept short because 9p mount_tag has a ~31-byte limit.
- Kernel cmdline: the full destination path is passed as
  slim.mount=<tag>:<base64(path)> so the init script knows where to
  mount each tag.  Base64 avoids issues with spaces/special chars.
- slim-init.sh: after networking, parse slim.mount= entries, mkdir -p
  the destination, and mount -t 9p <tag> <dest> -o trans=virtio,version=9p2000.L

Tests verify: 9p share detection via sysfs mount_tag, 9p entry in
mount output, file content accessible at the expected path, and clean
VM exit.

src/qemu.rs

@@ -32,6 +32,13 @@ pub struct RunCmd {
     /// cmdline as slim.cmd=<b64>). Overrides the CMD inferred at build time.
     #[clap(long)]
     cmd: Option<String>,
+
+    /// Share a host directory into the VM via 9p. Format:
+    /// `<host-path>` or `<host-path>:<guest-path>`. When the guest path is
+    /// omitted, the host path is used. Non-absolute guest paths are
+    /// relative to the image's WORKINGDIR. Can be repeated.
+    #[clap(long)]
+    mount: Vec<String>,
 }
 
 pub(crate) fn run(
@@ -40,6 +47,7 @@ pub(crate) fn run(
         memory,
         forward,
         cmd,
+        mount,
     }: RunCmd,
 ) -> Result<()> {
     let reg_dir = registry_dir(&name)?;
@@ -81,6 +89,35 @@ pub(crate) fn run(
         cmdline.push(format!("slim.cmd={encoded}"));
     }
 
+    // Build 9p shares for each --mount.  Tags are short (slim0, slim1, …)
+    // because 9p mount_tag has a ~31-byte limit.  The guest destination path
+    // is passed on the kernel cmdline as slim.mount=<tag>:<base64(guest_path)>.
+    // If no guest path is specified, the host path is used as the guest path.
+    let mut virtfs_args: Vec<String> = Vec::new();
+    for (i, spec) in mount.iter().enumerate() {
+        let (host_path, guest_path) = match spec.split_once(':') {
+            Some((h, g)) => (h, g),
+            None => (spec.as_str(), spec.as_str()),
+        };
+        let canonical = std::path::Path::new(host_path)
+            .canonicalize()
+            .with_context(|| format!("Cannot resolve mount path '{host_path}'"))?;
+        let host_str = canonical
+            .to_str()
+            .context("Mount path is not valid UTF-8")?;
+        let tag = format!("slim{i}");
+        let dest_b64 = base64::engine::general_purpose::STANDARD.encode(guest_path.as_bytes());
+
+        // QEMU's QemuOpts splits on commas — escape literal commas in the
+        // path as ",," per QEMU convention.
+        let host_escaped = host_str.replace(',', ",,");
+        virtfs_args.push("-virtfs".into());
+        virtfs_args.push(format!(
+            "local,path={host_escaped},mount_tag={tag},security_model=mapped-xattr"
+        ));
+        cmdline.push(format!("slim.mount={tag}:{dest_b64}"));
+    }
+
     println!("Booting {name} from registry: {}", reg_dir.display());
 
     let cmdline = cmdline.join(" ");
@@ -101,6 +138,7 @@ pub(crate) fn run(
         .arg("-kernel")
         .arg(vmlinuz_path.as_os_str())
         .args(&fs_args)
+        .args(&virtfs_args)
         .args(["-snapshot"])
         .args(["-no-reboot"])
         .args(["-append", &cmdline])