job: Add job_drain()

block_job_drain() contains a blk_drain() call which cannot be moved to
Job, so add a new JobDriver callback JobDriver.drain which has a common
implementation for all BlockJobs. In addition to this we keep the
existing BlockJobDriver.drain callback that is called by the common
drain implementation for all block jobs.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

block/backup.c

@@ -529,6 +529,7 @@ static const BlockJobDriver backup_job_driver = {
         .job_type               = JOB_TYPE_BACKUP,
         .free                   = block_job_free,
         .user_resume            = block_job_user_resume,
+        .drain                  = block_job_drain,
         .start                  = backup_run,
         .commit                 = backup_commit,
         .abort                  = backup_abort,