setup-disk: Install to virtiofs

When attempting to use root on virtiofs we get this error:

localhost:~# setup-disk /mnt
ERROR: unable to select packages:
  u-boot (no such package):
    required by: world[u-boot]
virtiofs is not supported. Only supported are: ext2 ext3 ext4 btrfs xfs vfat

The goal here is to be able to use virtiofs as the root directory.

https://virtio-fs.gitlab.io/howto-boot.html

We might need more than just allowing virtiofs here, since in my environment running

mount -t virtiofs share /mnt

Creates /mnt/alpine, where alpine is the name of the shared directory in the host.