qemu: Remove disk backend only after frontend is gone

qemu: Remove disk backend only after frontend is gone

In general, we should only remove a backend after seeing DEVICE_DELETED
event for a corresponding frontend. This doesn't make any difference for
disks attached using -drive or drive_add since QEMU automatically
removes their backends but it's still better to make our code
consistent. And it may start making difference in case we switch to
attaching disks using -blockdev.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>