kvm tools: Use '-d' to boot a directory as a rootfs

Pointing to a directory when specifying an image ('-d') will attempt to boot
the directory as if it was a root device using virtio-9p.

Tested-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>