job: Add reference counting

This moves reference counting from BlockJob to Job.

In order to keep calling the BlockJob cleanup code when the job is
deleted via job_unref(), introduce a new JobDriver.free callback. Every
block job must use block_job_free() for this callback, this is asserted
in block_job_create().

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

block/commit.c

@@ -218,6 +218,7 @@ static const BlockJobDriver commit_job_driver = {
     .job_driver = {
         .instance_size = sizeof(CommitBlockJob),
         .job_type      = JOB_TYPE_COMMIT,
+        .free          = block_job_free,
     },
     .start         = commit_run,
 };