qapi: Add device ID and head parameters to screendump

QEMU's screendump command can only take dumps from the primary display.
When using multiple VGA cards, there is no way to get a dump from a
secondary card or other display heads yet. So let's add a 'device' and
a 'head' parameter to the HMP and QMP commands to be able to specify
alternative devices and heads with the screendump command, too.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1520267868-31778-1-git-send-email-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
diff --git a/hmp-commands.hx b/hmp-commands.hx
index 964eb51..1723cbe 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -253,9 +253,10 @@
 
     {
         .name       = "screendump",
-        .args_type  = "filename:F",
-        .params     = "filename",
-        .help       = "save screen into PPM image 'filename'",
+        .args_type  = "filename:F,device:s?,head:i?",
+        .params     = "filename [device [head]]",
+        .help       = "save screen from head 'head' of display device 'device' "
+                      "into PPM image 'filename'",
         .cmd        = hmp_screendump,
     },