quorum: Fix crash in quorum_aio_cb()

quorum_aio_cb() emits the QUORUM_REPORT_BAD event if there's
an I/O error in a Quorum child. However sacb->aiocb must be
correctly initialized for this to happen. read_quorum_children() and
read_fifo_child() are not doing this, which results in a QEMU crash.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 8138570d071ba7e25db3736979234a1fd71dbd05.1457610443.git.berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
