Reimplement GPU_group.get_allowed_VGPU_types

We start with an empty set of VGPU types, and for each PGPU in the group
we fold its enabled VGPU types into the set. We end up with the set of
all VGPU types which are enabled on at least one of the PGPUs in the
group.

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