qapi: add query-cpu-definitions command (v2)

This command attempts to map to the behavior of -cpu ?.  Unfortunately, the
output of this command differs wildly across targets.

To accommodate this, we use a weak symbol to implement a default version of the
command that fails with a QERR_NOT_SUPPORTED error code.  Targets can then
override and implement this command if it makes sense for them.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/qmp-commands.hx b/qmp-commands.hx
index fd87775..521da10 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -2230,3 +2230,9 @@
         .mhandler.cmd_new = qmp_marshal_input_query_machines,
     },
 
+    {
+        .name       = "query-cpu-definitions",
+        .args_type  = "",
+        .mhandler.cmd_new = qmp_marshal_input_query_cpu_definitions,
+    },
+