QMP: add inject-nmi qmp command

inject-nmi command injects an NMI on all CPUs of guest.
It is only supported for x86 guest currently, it will
returns "Unsupported" error for non-x86 guest.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

qmp-commands.hx

@@ -427,6 +427,33 @@ Example:
                             "filename": "/tmp/physical-mem-dump" } }
 <- { "return": {} }
 
+EQMP
+
+    {
+        .name       = "inject-nmi",
+        .args_type  = "",
+        .params     = "",
+        .help       = "",
+        .user_print = monitor_user_noop,
+        .mhandler.cmd_new = do_inject_nmi_all,
+    },
+
+SQMP
+inject-nmi
+----------
+
+Inject an NMI on guest's CPUs.
+
+Arguments: None.
+
+Example:
+
+-> { "execute": "inject-nmi" }
+<- { "return": {} }
+
+Note: inject-nmi is only supported for x86 guest currently, it will
+      returns "Unsupported" error for non-x86 guest.
+
 EQMP
 
     {