Removing LXD VM breaks further refresh

If a machine which is a VM in a registered LXD VM host is removed from maas, trying to refresh the VM host will fail with the following error:

{'__all__': ['Virtual machine with this Bmc and Identifier already exists.']}

The reason is that the VirtualMachine that was previously linked to the machine is left around in MAAS (which is intended as the VM actually still exists in LXD).

When refreshing, maas should look first if a VirtualMachine already exists for the VM and update it as needed, linking it to the new machine.

Also, it seems maas currently doesn't remove VirtualMachines that have been removed from LXD during a refresh (unless they're linked to a Node).