qemuDomainAttachNetDevice: Support attach of type="user"

https://bugzilla.redhat.com/show_bug.cgi?id=1420668

This has worked in previous releases.
My commit c266b604 broke it.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_hotplug.c

@@ -1104,6 +1104,9 @@ qemuDomainAttachNetDevice(virQEMUDriverPtr driver,
         break;
 
     case VIR_DOMAIN_NET_TYPE_USER:
+        /* No preparation needed. */
+        break;
+
     case VIR_DOMAIN_NET_TYPE_SERVER:
     case VIR_DOMAIN_NET_TYPE_CLIENT:
     case VIR_DOMAIN_NET_TYPE_MCAST: