network: explicitly set the MTU of the bridge device.

In the past, the MTU of libvirt virtual network bridge devices was
implicitly set by setting the MTU of the "dummy tap device" (which was
being added in order to force a particular MAC address from the
bridge). But the dummy tap device was removed in commit ee6c936fbb
(libvirt-6.8.0), and so the mtu setting in the network is ignored.

The solution is, of course, to explicitly set the bridge device MTU
when it is created.

Note that any guest interface with a larger MTU that is attached will
cause the bridge to (temporarily) assume the larger MTU, but it will
revert to the bridge's own MTU when that device is deleted (this is
not due to anything libvirt does; it's just how Linux host bridges
work).

Fixes: ee6c936fbb
Resolves: https://bugzilla.redhat.com/1913561
Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/network/bridge_driver.c

@@ -2336,6 +2336,9 @@ networkStartNetworkVirtual(virNetworkDriverStatePtr driver,
 
     /* Set bridge options */
 
+    if (def->mtu && virNetDevSetMTU(def->bridge, def->mtu) < 0)
+        goto error;
+
     /* delay is configured in seconds, but virNetDevBridgeSetSTPDelay
      * expects milliseconds
      */