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; each host directory
appears at the same absolute path inside the VM.

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

@@ -8,7 +8,7 @@
 use anyhow::{Context, Result, bail};
 use base64::Engine;
 use clap::Args;
-use std::{fmt::Write as _, process::Command};
+use std::{fmt::Write as _, path::PathBuf, process::Command};
 
 use crate::{
     forward::PortForward,
@@ -32,6 +32,11 @@ 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. The directory appears at
+    /// the same path inside the VM. Can be repeated for multiple mounts.
+    #[clap(long)]
+    mount: Vec<PathBuf>,
 }
 
 pub(crate) fn run(
@@ -40,6 +45,7 @@ pub(crate) fn run(
         memory,
         forward,
         cmd,
+        mount,
     }: RunCmd,
 ) -> Result<()> {
     let reg_dir = registry_dir(&name)?;
@@ -81,6 +87,27 @@ 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 full destination path
+    // is passed on the kernel cmdline as slim.mount=<tag>:<base64(path)>.
+    let mut virtfs_args: Vec<String> = Vec::new();
+    for (i, host_path) in mount.iter().enumerate() {
+        let canonical = host_path
+            .canonicalize()
+            .with_context(|| format!("Cannot resolve mount path '{}'", host_path.display()))?;
+        let path_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(path_str.as_bytes());
+
+        virtfs_args.push("-virtfs".into());
+        virtfs_args.push(format!(
+            "local,path={path_str},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 +128,7 @@ 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])