migration: Use error_reportf_err() instead of monitor_printf()

Both error_reportf_err() and monitor_printf() print to the same
destination when monitor_printf() is used correctly, i.e. within an
HMP monitor.  Elsewhere, monitor_printf() does nothing, while
error_reportf_err() reports to stderr.

Both changed functions are HMP command handlers.  These should only
run within an HMP monitor.

Unlike monitor_printf(), error_reportf_err() uses the error whole
instead of just its message obtained with error_get_pretty().  This
avoids suppressing its hint (see commit 50b7b00), but I don't think
the errors touched in this commit can come with hints.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-15-git-send-email-armbru@redhat.com>

migration/savevm.c

@@ -1927,10 +1927,9 @@ void hmp_savevm(Monitor *mon, const QDict *qdict)
 
     /* Delete old snapshots of the same name */
     if (name && bdrv_all_delete_snapshot(name, &bs1, &local_err) < 0) {
-        monitor_printf(mon,
-                       "Error while deleting snapshot on device '%s': %s\n",
-                       bdrv_get_device_name(bs1), error_get_pretty(local_err));
-        error_free(local_err);
+        error_reportf_err(local_err,
+                          "Error while deleting snapshot on device '%s': ",
+                          bdrv_get_device_name(bs1));
         return;
     }
 
@@ -2108,10 +2107,9 @@ void hmp_delvm(Monitor *mon, const QDict *qdict)
     const char *name = qdict_get_str(qdict, "name");
 
     if (bdrv_all_delete_snapshot(name, &bs, &err) < 0) {
-        monitor_printf(mon,
-                       "Error while deleting snapshot on device '%s': %s\n",
-                       bdrv_get_device_name(bs), error_get_pretty(err));
-        error_free(err);
+        error_reportf_err(err,
+                          "Error while deleting snapshot on device '%s': ",
+                          bdrv_get_device_name(bs));
     }
 }