x86/vlapic: tidy switch statement and address MISRA violation

Refactor the last clauses so that a violation of
MISRA C Rule 16.2 is resolved (A switch label shall only be used
when the most closely-enclosing compound statement is the body of
a switch statement). The switch clause ending with the
pseudo keyword "fallthrough" is an allowed exception to Rule 16.3.

No functional change.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/x86/hvm/vlapic.c

@@ -1033,11 +1033,11 @@ int guest_wrmsr_x2apic(struct vcpu *v, uint32_t msr, uint64_t val)
 
     case APIC_EOI:
     case APIC_ESR:
-        if ( val )
-        {
+        if ( !val )
+            break;
+        fallthrough;
     default:
             return X86EMUL_EXCEPTION;
-        }
     }
 
     vlapic_reg_write(v, array_index_nospec(offset, PAGE_SIZE), val);