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>