Add support for tracking thread jobs

Add support for tracking thread jobs

Each thread can use a thread local variable to keep the name of a job
which is currently running in the job.

The virThreadJobSetWorker API is supposed to be called once by any
thread which is used as a worker, i.e., it is waiting in a pool, woken
up to do a job, and returned back to the pool.

The virThreadJobSet/virThreadJobClear APIs are to be called at the
beginning/end of each job.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>