do_drive_del()'s code to clean up the pointer from a qdev using the
drive back to the drive needs to check whether such a device exists.
Signed-off-by: Markus Armbruster <armbru@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>