block/core: add generic infrastructure for x-blockdev-amend qmp command

blockdev-amend will be used similiar to blockdev-create
to allow on the fly changes of the structure of the format based block devices.

Current plan is to first support encryption keyslot management for luks
based formats (raw and embedded in qcow2)

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200608094030.670121-12-mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/qapi/job.json b/qapi/job.json
index 5e65828..c48a0c3 100644
--- a/qapi/job.json
+++ b/qapi/job.json
@@ -19,10 +19,12 @@
 #
 # @create: image creation job type, see "blockdev-create" (since 3.0)
 #
+# @amend: image options amend job type, see "x-blockdev-amend" (since 5.1)
+#
 # Since: 1.7
 ##
 { 'enum': 'JobType',
-  'data': ['commit', 'stream', 'mirror', 'backup', 'create'] }
+  'data': ['commit', 'stream', 'mirror', 'backup', 'create', 'amend'] }
 
 ##
 # @JobStatus: