qapi: convert screendump

Next commits will update devices to propagate errors.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/qapi-schema.json b/qapi-schema.json
index 5421382..df47fce 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -2539,3 +2539,16 @@
 ##
 { 'command': 'send-key',
   'data': { 'keys': ['QKeyCode'], '*hold-time': 'int' } }
+
+##
+# @screendump:
+#
+# Write a PPM of the VGA screen to a file.
+#
+# @filename: the path of a new PPM file to store the image
+#
+# Returns: Nothing on success
+#
+# Since: 0.14.0
+##
+{ 'command': 'screendump', 'data': {'filename': 'str'} }