support running via virtiofs

[cgwalters]

https://gitlab.com/bootc-org/podman-bootc-cli/-/issues/31

It'd be nice to have podman-bootc run --virtiofs <container> that would set things up to boot via virtiofs root. I think to really make this work sanely, we'd need to move the container storage out of the podman-machine VM and on to the host. That would have all sorts of implications.

Failing that, maybe we could do run --nfs to set up a NFS-backed rootfs talking to a NFS server serving the container storage on the target.
I think composefs will help here in both cases, as we can have all the selinux labels and metadata set up in the composefs, and have the virtiofs/nfs be a lower FS just fetching the backing files.

Either way steps would also include

• Extracting kernel/initramfs from the container
• Setting up qemu to launch things that way