Rename GPU_group.get_allowed_VGPU_types to get_enabled_VGPU_types

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

Refactor part of GPU_group.get_enabled_VGPU_types so it can be reused.

Signed-off-by: John Else <john.else@citrix.com>
Add GPU_group.supported_VGPU_types to the CLI

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1400 from johnelse/gpu-group-methods

Implement new GPU_group methods