vfio-pci: Add module option to disable VGA region access

Add a module option so that we don't require a CONFIG change and
kernel rebuild to disable VGA support.  Not only can VGA support be
troublesome in itself, but by disabling it we can reduce the impact
to host devices by doing a VGA arbitration opt-out.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>