Empty file used to create an empty drive (no media).  Since commit
9dfd7c7a

, it's an error: "qemu: could not open disk image : No such
file or directory".  Older versions of libvirt can choke on this.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>