Dump: add qmp command "query-dump"

When dump-guest-memory is requested with detach flag, after its
return, user could query its status using "query-dump" command (with
no argument). The result contains:

- status: current dump status
- completed: bytes written in the latest dump
- total: bytes to write in the latest dump

From completed and total, we could know how much work
finished by calculating:

  100.0 * completed / total (%)

Reviewed-by:   Fam Zheng <famz@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1455772616-8668-10-git-send-email-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qmp-commands.hx

@@ -856,7 +856,8 @@ Arguments:
 - "protocol": destination file(started with "file:") or destination file
               descriptor (started with "fd:") (json-string)
 - "detach": if specified, command will return immediately, without waiting
-            for the dump to finish (json-bool)
+            for the dump to finish. The user can track progress using
+            "query-dump". (json-bool)
 - "begin": the starting physical address. It's optional, and should be specified
            with length together (json-int)
 - "length": the memory size, in bytes. It's optional, and should be specified
@@ -894,6 +895,30 @@ Example:
 <- { "return": { "formats":
                     ["elf", "kdump-zlib", "kdump-lzo", "kdump-snappy"] }
 
+EQMP
+
+    {
+        .name       = "query-dump",
+        .args_type  = "",
+        .params     = "",
+        .help       = "query background dump status",
+        .mhandler.cmd_new = qmp_marshal_query_dump,
+    },
+
+SQMP
+query-dump
+----------
+
+Query background dump status.
+
+Arguments: None.
+
+Example:
+
+-> { "execute": "query-dump" }
+<- { "return": { "status": "active", "completed": 1024000,
+                 "total": 2048000 } }
+
 EQMP
 
 #if defined TARGET_S390X