job: Add JobDriver.job_type

This moves the job_type field from BlockJobDriver to JobDriver.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
diff --git a/block/backup.c b/block/backup.c
index c49ea92..baf8d43 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -525,8 +525,8 @@
 static const BlockJobDriver backup_job_driver = {
     .job_driver = {
         .instance_size          = sizeof(BackupBlockJob),
+        .job_type               = JOB_TYPE_BACKUP,
     },
-    .job_type               = JOB_TYPE_BACKUP,
     .start                  = backup_run,
     .commit                 = backup_commit,
     .abort                  = backup_abort,