jobs: protect job.aio_context with BQL and job_mutex

jobs: protect job.aio_context with BQL and job_mutex

In order to make it thread safe, implement a "fake rwlock",
where we allow reads under BQL *or* job_mutex held, but
writes only under BQL *and* job_mutex.

The only write we have is in child_job_set_aio_ctx, which always
happens under drain (so the job is paused).
For this reason, introduce job_set_aio_context and make sure that
the context is set under BQL, job_mutex and drain.
Also make sure all other places where the aiocontext is read
are protected.

The reads in commit.c and mirror.c are actually safe, because always
done under BQL.

Note: at this stage, job_{lock/unlock} and job lock guard macros
are *nop*.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20220926093214.506243-14-eesposit@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>