Loosen restriction on PGPU.add_enabled_VGPU_types

Allow adding enabled VGPU types even if there are running VGPUs on the
PGPU.

Signed-off-by: John Else <john.else@citrix.com>
Loosen restriction on PGPU.remove_enabled_VGPU_types

Allow removing enabled VGPU types from a PGPU if there are no running
VGPUs of that type.

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