block: let mirror blockjob run in BDS AioContext

The mirror block job must run in the BlockDriverState AioContext so that
it works with dataplane.

Acquire the AioContext in blockdev.c so starting the block job is safe.

Note that to_replace is treated separately from other BlockDriverStates
in that it does not need to be in the same AioContext.  Explicitly
acquire/release to_replace's AioContext when accessing it.

The completion code in block/mirror.c must perform BDS graph
manipulation and bdrv_reopen() from the main loop.  Use
block_job_defer_to_main_loop() to achieve that.

The bdrv_drain_all() call is not allowed outside the main loop since it
could lead to lock ordering problems.  Use bdrv_drain(bs) instead
because we have acquired the AioContext so nothing else can sneak in
I/O.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1413889440-32577-10-git-send-email-stefanha@redhat.com
