virDomainNetDefCheckABIStability: Check for MTU change too

https://bugzilla.redhat.com/show_bug.cgi?id=1623157

Changing MTU on a running guest is not possible and trying to do
so made us face many problems. That's why we forbid it in
5f44d7e357. However, there is still one possible path where
users can sneak in change: migration XML.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

src/conf/domain_conf.c

@@ -21901,6 +21901,13 @@ virDomainNetDefCheckABIStability(virDomainNetDefPtr src,
         return false;
     }
 
+    if (src->mtu != dst->mtu) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                       _("Target network card MTU %d does not match source %d"),
+                       dst->mtu, src->mtu);
+        return false;
+    }
+
     if (src->virtio && dst->virtio &&
         !virDomainVirtioOptionsCheckABIStability(src->virtio, dst->virtio))
         return false;