block/drive-mirror: Check for NULL backing_hd

It should be possible to execute the QMP "drive-mirror" command in
"none" sync mode and "absolute-paths" mode even for block devices
lacking a backing file.

"absolute-paths" does in fact not require a backing file to be present,
as can be seen from the "top" sync mode code path. "top" basically
states that the device should indeed have a backing file - however, the
current code catches the case if it doesn't and then simply treats it as
"full" sync mode, creating a target image without a backing file (in
"absolute-paths" mode). Thus, "absolute-paths" does not imply the target
file must indeed have a backing file.

Therefore, the target file may be left unbacked in case of "none" sync
mode as well, if the specified device is not backed either. Currently,
qemu will crash trying to dereference the backing file pointer since it
assumes that it will always be non-NULL in that case ("none" with
"absolute-paths").

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
