details: Check for mac conflict in customize wizard

https://bugzilla.redhat.com/show_bug.cgi?id=1660467

virtManager/details.py

@@ -1791,6 +1791,8 @@ class vmmDetails(vmmGObjectUI):
 
         if self.edited(EDIT_NET_MAC):
             kwargs["macaddr"] = self.widget("network-mac-entry").get_text()
+            virtinst.DeviceInterface.is_conflict_net(
+                    self.conn.get_backend(), kwargs["macaddr"])
 
         if self.edited(EDIT_NET_LINKSTATE):
             kwargs["linkstate"] = self.widget("network-link-state-checkbox").get_active()