blockjob: add pause points

Block jobs are coroutines that usually perform I/O but sometimes also
sleep or yield.  Currently only sleeping or yielded block jobs can be
paused.  This means jobs that do not sleep or yield (using
block_job_yield()) are unaffected by block_job_pause().

Add block_job_pause_point() so that block jobs can mark quiescent points
that are suitable for pausing.  This solves the problem that it can take
a block job a long time to pause if it is performing a long series of
I/O operations.

Transitioning to paused state involves a .pause()/.resume() callback.
These callbacks are used to ensure that I/O and event loop activity has
ceased while the job is at a pause point.

Note that this patch introduces a stricter pause state than previously.
The job->busy flag was incorrectly documented as a quiescent state
without I/O pending.  This is violated by any job that has I/O pending
across sleep or block_job_yield(), like the mirror block job.

[Add missing block_job_should_pause() check to avoid deadlock after
job->driver->pause() in block_job_pause_point().
--Stefan]

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1466096189-6477-4-git-send-email-stefanha@redhat.com
