qapi: add query-display-options command

Add query-display-options command, which allows querying the qemu
display configuration.  This isn't particularly useful, except it
exposes QAPI type DisplayOptions in query-qmp-schema, so that libvirt
can discover recently added -display parameter rendernode (commit
d4dc4ab1).  Works around lack of sufficiently powerful command line
introspection.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Tested-by: NEric Blake <eblake@redhat.com>
Tested-by: NErik Skultety <eskultet@redhat.com>
Message-id: 20181122071613.2889-1-kraxel@redhat.com

[ kraxel: reworded commit message as suggested by armbru ]

qapi/ui.json

@@ -1102,3 +1102,16 @@
   'discriminator' : 'type',
   'data'    : { 'gtk'            : 'DisplayGTK',
                 'egl-headless'   : 'DisplayEGLHeadless'} }
+
+##
+# @query-display-options:
+#
+# Returns information about display configuration
+#
+# Returns: @DisplayOptions
+#
+# Since: 3.1
+#
+##
+{ 'command': 'query-display-options',
+  'returns': 'DisplayOptions' }

vl.c

@@ -128,6 +128,7 @@ int main(int argc, char **argv)
 #include "qapi/qapi-commands-block-core.h"
 #include "qapi/qapi-commands-misc.h"
 #include "qapi/qapi-commands-run-state.h"
+#include "qapi/qapi-commands-ui.h"
 #include "qapi/qmp/qerror.h"
 #include "sysemu/iothread.h"
 
@@ -2055,6 +2056,11 @@ static void parse_display_qapi(const char *optarg)
     visit_free(v);
 }
 
+DisplayOptions *qmp_query_display_options(Error **errp)
+{
+    return QAPI_CLONE(DisplayOptions, &dpy);
+}
+
 static void parse_display(const char *p)
 {
     const char *opts;