Change behaviour of VGPU_type import lookup

When attempting to import a VGPU_type, look for the equivalent existing
VGPU_type using the (model_name,vendor_name) tuple, rather than
(model_name,framebuffer_size).

This matches the lookup in Xapi_vgpu_type.find_or_create

Signed-off-by: John Else <john.else@citrix.com>
CA-121019: Loosen restrictions on VGPU_type import

When importing a VGPU_type which doesn't exist locally, import the
record instead of failing the import. We'll end up with a VGPU which has
this type, but no PGPUs which support this type, so the VM with the VGPU
will be unable to start.

The VM will be able to start once a PGPU which supports the type is
available.

Signed-off-by: John Else <john.else@citrix.com>
CA-122441: Update group supported/enabled types after GCing PGPUs

Signed-off-by: John Else <john.else@citrix.com>
Log every time we GC a PGPU

Signed-off-by: John Else <john.else@citrix.com>
Add new fields to make_vgpu_type

Signed-off-by: John Else <john.else@citrix.com>
Improve consistency of test_common.ml

Fix all the make_* functions such that they return the ref of the thing
which they created.

Signed-off-by: John Else <john.else@citrix.com>
CA-122441: Add unit test for GPU_group supported/enabled VGPU types

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