virtiofsd will not migrate/move/copy the file system content to the
target (and most probably will never do it). But this migration
mode implies migrating the file system content.

The target FS will be empty and virtiofsd will successfully migrate
to an empty target directory, returning an error to the guest when
the guest tries to access a file that no longer exists.

But after talking with Adam's team, we think is better to not
allow storage migration if virtiofsd is used.