feat: add --kernel flag for QEMU system emulation mode

Add --kernel <PATH> option to boot extracted rootfs in a QEMU virtual
machine instead of namespace/chroot mode. The rootfs is converted to an
ext4 disk image using mke2fs and booted with the provided kernel.

src/cli.rs

@@ -33,6 +33,14 @@ pub struct Args {
     #[arg(short = 'v', long)]
     pub verbose: bool,
 
+    /// Boot with QEMU system emulation using specified kernel (extracts rootfs as disk image)
+    #[arg(long, value_name = "KERNEL_PATH")]
+    pub kernel: Option<PathBuf>,
+
+    /// Memory size for QEMU VM (only used with --kernel, e.g., 512M, 2G)
+    #[arg(short = 'm', long, default_value = "2G", value_name = "SIZE")]
+    pub memory: String,
+
     /// Command to run inside the chroot (default: interactive shell)
     #[arg(
         trailing_var_arg = true,