CA-121671: Tighten up PGPU.set_enabled_VGPU_types

The PGPU.remove_enabled_VGPU_types API checked if there were any VMs running of
that type before disabling the type. However, the set_enabled_VGPU_types call
(which may disable some types) made no such check.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Correct field name in assert_no_resident_VGPUs_of_type

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>