qga: Tweak a guest-shutdown error message

Change

    Parameter 'mode' expects halt|powerdown|reboot

to

    Parameter 'mode' expects 'halt', 'powerdown', or 'reboot'

for consistency with similar error messages elsewhere.

Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201113082626.2725812-9-armbru@redhat.com>

qga/commands-win32.c

@@ -334,7 +334,7 @@ void qmp_guest_shutdown(bool has_mode, const char *mode, Error **errp)
         shutdown_flag |= EWX_REBOOT;
     } else {
         error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "mode",
-                   "halt|powerdown|reboot");
+                   "'halt', 'powerdown', or 'reboot'");
         return;
     }