Migratable virtiofs

@pipo.sk outlined this better on the libvirt repo, the following is a patch to note this deficiency:

virtiofs currently does not support migration so operations such as migration, save/managed-save, or snapshots with memory are not supported if a VM has a virtiofs filesystem connected.

Since it is relevant, I'll also quote this from them:

The virtiofs device indeed doesn't currently support migration (it's non-trivial and has a lot of caveats). Migration is used also in case of save/managedsave operations to serialize the qemu state to disk, thus using a virtiofs device will indeed prevent you from using the features needing migration.

It was suggested to open a feature request here, as it seems both of us could not find a specific related issue.