qemu: Set iface MTU on hotplug

https://bugzilla.redhat.com/show_bug.cgi?id=1408701

While implementing MTU (572eda12 and friends), I've forgotten
to actually set MTU on the host NIC in case of hotplug. We
correctly tell qemu on the monitor what the MTU should be, but we
are not actually setting it on the host NIC.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NLaine Stump <laine@laine.org>
(cherry picked from commit f00e6f8b)

src/qemu/qemu_hotplug.c

@@ -1134,6 +1134,10 @@ qemuDomainAttachNetDevice(virQEMUDriverPtr driver,
         }
     }
 
+    if (net->mtu &&
+        virNetDevSetMTU(net->ifname, net->mtu) < 0)
+        goto cleanup;
+
     for (i = 0; i < tapfdSize; i++) {
         if (qemuSecuritySetTapFDLabel(driver->securityManager,
                                       vm->def, tapfd[i]) < 0)