virtio-fs: expose read-only shared directories

[claude-for-evoskamp]

VZSharedDirectory supports readOnly, but pkg/vf/virtio.go always passes false (vz.NewSharedDirectory(dev.SharedDir, false)), and the virtio-fs device rejects any read-only option (Error: unknown option for virtio-fs devices: readOnly, v0.6.4). Block devices already accept readonly.

Please add a readonly option to --device virtio-fs (and the JSON config) and pass it to NewSharedDirectory. This gives guests a live view of a host directory where writes are refused by the hypervisor, so guest root can't bypass it the way it can a guest-side mount -o ro.