CP-5597: Raise error when attempting to create vGPU without allowed VGPU_type

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-5597: Gracefully handle missing nVidia config files

This would be the case if you hadn't installed the RPM or didn't want to use
vGPU functionality.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>