qmp: Add optional bool "unmap" to drive-mirror

If specified as "true", it allows discarding on target sectors where source is
not allocated.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 87fa172..e1bcc60 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -1503,6 +1503,7 @@
         .args_type  = "sync:s,device:B,target:s,speed:i?,mode:s?,format:s?,"
                       "node-name:s?,replaces:s?,"
                       "on-source-error:s?,on-target-error:s?,"
+                      "unmap:b?,"
                       "granularity:i?,buf-size:i?",
         .mhandler.cmd_new = qmp_marshal_input_drive_mirror,
     },
@@ -1542,6 +1543,8 @@
   (BlockdevOnError, default 'report')
 - "on-target-error": the action to take on an error on the target
   (BlockdevOnError, default 'report')
+- "unmap": whether the target sectors should be discarded where source has only
+  zeroes. (json-bool, optional, default true)
 
 The default value of the granularity is the image cluster size clamped
 between 4096 and 65536, if the image format defines one.  If the format