CP-5812: 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>
CP-5812: Add relation VGPU.resident_on <-> PGPU.resident_VGPUs.

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