x86/hvm: address violations of MISRA C Rule 16.2

Refactor the switch so that a violation of
MISRA C Rule 16.2 is resolved (a switch label should be immediately
enclosed in the compound statement of the switch).

The switch clause ending with the pseudo
keyword "fallthrough" is an allowed exception to Rule 16.3.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/hypercall.c

@@ -119,12 +119,12 @@ int hvm_hypercall(struct cpu_user_regs *regs)
             (mode == 8 ? regs->rdi : regs->ebx) == HVMOP_guest_request_vm_event )
             break;
 
-        if ( unlikely(hvm_get_cpl(curr)) )
-        {
+        if ( likely(!hvm_get_cpl(curr)) )
+            break;
+        fallthrough;
     default:
-            regs->rax = -EPERM;
-            return HVM_HCALL_completed;
-        }
+        regs->rax = -EPERM;
+        return HVM_HCALL_completed;
     case 0:
         break;
     }