This patch allows an empty filename to be passed as the new base image name
for qemu-img rebase to mean base the image on no backing file (i.e.
independent of any backing file). According to Eric Blake, qemu-img rebase
already supports this when '-u' is used; this adds support when -u is not
used.
Signed-off-by: NAlex Bligh <alex@alex.org.uk>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>