xen/vm-event: address a violation of MISRA C:2012 Rule 16.3

Add break statement to address a violation of MISRA C:2012 Rule 16.3
("An unconditional `break' statement shall terminate every
switch-clause ").

No functional change.

Signed-off-by: Federico Serafini <federico.serafini@bugseng.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>

xen/common/vm_event.c

@@ -770,6 +770,7 @@ int vm_event_domctl(struct domain *d, struct xen_domctl_vm_event_op *vec)
 
     default:
         rc = -ENOSYS;
+        break;
     }
 
     return rc;