qemu: Set iface MTU on hotplug

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

While implementing MTU (572eda12ad 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: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Laine Stump <laine@laine.org>

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)