VM device selection does not filter the list by the selected site

[abhi1693]

NetBox version

v3.3.0-beta1

Python version

3.9

Steps to Reproduce

1. Create a new VM named vm1 and associate it to site DM-Akron and click Save
2. Edit the same VM and click on the Device drop-down and choose the 1st entry from the list and click save

Expected Behavior

The device list should be pre-filtered by the already associated site

Observed Behavior

An error is thrown (this is good to have as validation but not so user-friendly approach)

[jeremystretch]

Edit the same VM and click on the Device drop-down and choose the 1st entry from the list and click save

The issue is that the devices is list is not being filtered by the assigned site; easy enough to fix.

However, there's a second issue, where even if a device within that site is selected, an AttributeError exception is raised because a device has been selected but no cluster is assigned. This is because we're missing a validation rule for the cluster.

I'm going to resolve both bugs under this issue as they're very closely related.