block: acquire AioContext in generic blockjob QMP commands

block-job-set-speed, block-job-cancel, block-job-pause,
block-job-resume, and block-job-complete must acquire the
BlockDriverState AioContext so that it is safe to access bs.

At the moment bs->job is always NULL when dataplane is active because op
blockers prevent blockjobs from starting.  Once the rest of the blockjob
API has been made aware of AioContext we can drop the op blocker.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1413889440-32577-2-git-send-email-stefanha@redhat.com
