qemuDomainChangeNet: Forbid changing MTU

https://bugzilla.redhat.com/show_bug.cgi?id=1447618

Currently, any attempt to change MTU on an interface that is
plugged to a running domain is silently ignored. We should either
do what's asked or error out. Well, we can update the host side
of the interface, but we cannot change 'host_mtu' attribute for
the virtio-net device. Therefore we have to error out.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Laine Stump <laine@laine.org>

src/qemu/qemu_hotplug.c

@@ -3138,6 +3138,12 @@ qemuDomainChangeNet(virQEMUDriverPtr driver,
     /* vlan can be modified, and will be checked later */
     /* linkstate can be modified */
 
+    if (olddev->mtu != newdev->mtu) {
+        virReportError(VIR_ERR_OPERATION_UNSUPPORTED, "%s",
+                       _("cannot modify MTU"));
+        goto cleanup;
+    }
+
     /* allocate new actual device to compare to old - we will need to
      * free it if we fail for any reason
      */