blockjob: Allow nested pause

blockjob: Allow nested pause

This patch changes block_job_pause to increase the pause counter and
block_job_resume to decrease it.

The counter will allow calling block_job_pause/block_job_resume
unconditionally on a job when we need to suspend the IO temporarily.

From now on, each block_job_resume must be paired with a block_job_pause
to keep the counter balanced.

The user pause from QMP or HMP will only trigger block_job_pause once
until it's resumed, this is achieved by adding a user_paused flag in
BlockJob.

One occurrence of block_job_resume in mirror_complete is replaced with
block_job_enter which does what is necessary.

In block_job_cancel, the cancel flag is good enough to instruct
coroutines to quit loop, so use block_job_enter to replace the unpaired
block_job_resume.

Upon block job IO error, user is notified about the entering to the
pause state, so this pause belongs to user pause, set the flag
accordingly and expect a matching QMP resume.

[Extended doc comments as suggested by Paolo Bonzini
<[email protected]>.
--Stefan]

Signed-off-by: Fam Zheng <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Reviewed-by: Alberto Garcia <[email protected]>
Message-id: 1428069921[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>