qapi: Introduce enum BlockJobType

This will replace the open coded block job type string for mirror,
commit and backup.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qapi-schema.json

@@ -1365,6 +1365,24 @@
 { 'enum': 'MirrorSyncMode',
   'data': ['top', 'full', 'none'] }
 
+##
+# @BlockJobType:
+#
+# Type of a block job.
+#
+# @commit: block commit job type, see "block-commit"
+#
+# @stream: block stream job type, see "block-stream"
+#
+# @mirror: drive mirror job type, see "drive-mirror"
+#
+# @backup: drive backup job type, see "drive-backup"
+#
+# Since: 1.7
+##
+{ 'enum': 'BlockJobType',
+  'data': ['commit', 'stream', 'mirror', 'backup'] }
+
 ##
 # @BlockJobInfo:
 #