blockdev: acquire AioContext in QMP 'transaction' actions

The transaction QMP command performs operations atomically on a group of
drives.  This command needs to acquire AioContext in order to work
safely when virtio-blk dataplane IOThreads are accessing drives.

The transactional nature of the command means that actions are split
into prepare, commit, abort, and clean functions.  Acquire the
AioContext in prepare and don't release it until one of the other
functions is called.  This prevents the IOThread from running the
AioContext before the transaction has completed.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1416566940-4430-4-git-send-email-stefanha@redhat.com
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
