CP-5842: Blank VGPU.resident_on on VM shutdown

Signed-off-by: John Else <john.else@citrix.com>
CP-5842: Update VGPU.resident_on for passthrough VGPUs

Signed-off-by: John Else <john.else@citrix.com>
Allow starting of VMs without virtual GPUs

Signed-off-by: John Else <john.else@citrix.com>