Add a relation between a PGPU and its enabled VGPU_types.

Signed-off-by: John Else <john.else@citrix.com>
Initialise a PGPU's enabled VGPU types on creation.

enabled_VGPU_types is initialised to be equal to supported_VGPU_types.

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