qga: Replace an unreachable error by abort()

check_suspend_mode()'s error message

    Parameter 'mode' expects GuestSuspendMode

makes no sense to users: GuestSuspendMode is a C enum.  Fortunately,
it is unreachable.  Replace it by abort().

Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201113082626.2725812-8-armbru@redhat.com>

qga/commands-win32.c

@@ -1441,8 +1441,7 @@ static void check_suspend_mode(GuestSuspendMode mode, Error **errp)
         }
         break;
     default:
-        error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "mode",
-                   "GuestSuspendMode");
+        abort();
     }
 }