virtio-blk: Fix use after free in error case

virtio_blk_req_complete frees the request, so we can't access it any more when
calling bdrv_mon_event. Use the pointer that was copied earlier.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
