vfio/pci: Cleanup Kconfig

It should be possible to select vfio-pci variant drivers without building
vfio-pci itself, which implies each variant driver should select
vfio-pci-core.

Fix the top level vfio Makefile to traverse pci based on vfio-pci-core
rather than vfio-pci.

Mark MMAP and INTX options depending on vfio-pci-core to cleanup resulting
config if core is not enabled.

Push all PCI related vfio options to a sub-menu and make descriptions
consistent.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20230614193948.477036-2-alex.williamson@redhat.com


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