domain_validate: Properly terminate switch() in virDomainIOMMUDefValidate()

In my previous commit I've introduced virDomainIOMMUDefValidate()
function with a switch() statement. However, two cases in it,
though empty, were not terminated with a break statement which
made compiler complain.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

src/conf/domain_validate.c

@@ -2598,6 +2598,7 @@ virDomainIOMMUDefValidate(const virDomainIOMMUDef *iommu)
 
     case VIR_DOMAIN_IOMMU_MODEL_VIRTIO:
     case VIR_DOMAIN_IOMMU_MODEL_LAST:
+        break;
     }
 
     return 0;