qemu-ga: guest-shutdown: don't emit a success response

Today, qemu-ga may not be able to emit a success response when
guest-shutdown completes. This happens because the VM may vanish
before qemu-ga is able to emit a response.

This semantic is a bit confusing, as it's not clear for clients if
they should wait for a response or how they should check for success.

This commit solves that problem by changing guest-shutdown to never
emit a success response and suggests in the documentation what
clients could do to check for success.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

qapi-schema-guest.json

@@ -131,11 +131,15 @@
 #
 # @mode: #optional "halt", "powerdown" (default), or "reboot"
 #
-# Returns: Nothing on success
+# This command does NOT return a response on success. Success condition
+# is indicated by the VM exiting with a zero exit status or, when
+# running with --no-shutdown, by issuing the query-status QMP command
+# to confirm the VM status is "shutdown".
 #
 # Since: 0.15.0
 ##
-{ 'command': 'guest-shutdown', 'data': { '*mode': 'str' } }
+{ 'command': 'guest-shutdown', 'data': { '*mode': 'str' },
+  'success-response': 'no' }
 
 ##
 # @guest-file-open: