Hide VNC option if VM has existing VNC device
Currently, we assign the VNC device to slot 29 in the middleware plugin, so if there are two (or more) VNC devices they clash as both cannot occupy the same port. If the VM already has a VNC device, the web UI should not allow them to add another one. Supporting multiple VNC devices is not allowed at this time.
Tested on latest nightlies (FreeNAS-11.2-MASTER-201807200857).
Method of testing:
1. Created a FreeBSD VM with the latest nightlies FreeNAS .iso
2. During the process I made sure the Enable VNC option was checked.
3. After creation I clicked the Add plus symbol and found that the VNC type was not in the drop down list. See attached screenshot.