trace: Add QAPI/QMP interfaces to query and control per-vCPU tracing state

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

hmp-commands.hx

@@ -281,9 +281,10 @@ ETEXI
 
     {
         .name       = "trace-event",
-        .args_type  = "name:s,option:b",
-        .params     = "name on|off",
-        .help       = "changes status of a specific trace event",
+        .args_type  = "name:s,option:b,vcpu:i?",
+        .params     = "name on|off [vcpu]",
+        .help       = "changes status of a specific trace event "
+                      "(vcpu: vCPU to set, default is all)",
         .mhandler.cmd = hmp_trace_event,
         .command_completion = trace_event_completion,
     },