Revert "mirror: Workaround for unexpected iohandler events during completion"

This reverts commit ab27c3b5e7408693dde0b565f050aa55c4a1bcef.

The virtio storage device host notifiers now work with
bdrv_drained_begin/end, so we don't need this hack any more.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
