CA-114551: Update VGPU_type if the config has changed

max_instances or max_heads may have changed since we last read the
config file. In future, we may also want to update internal_config at
this point.

Signed-off-by: John Else <john.else@citrix.com>
CA-114551: Allow framebuffer_size to change

framebuffer_size used to be one of the invariants which we used to look
up existing VGPU types - now we update the existing VGPU type with the
new framebuffer_size.

Signed-off-by: John Else <john.else@citrix.com>
CA-114551: Add unit test

find_or_create-ing a VGPU type, then find_or_create-ing the same type
but with modified framebuffer_size and/or max_heads and/or size, should
result in the original VGPU type being updated with the new values.

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