vfio/pci: Rename vfio_pci_register_dev_region()

As this is part of the vfio_pci_core component it should be called
vfio_pci_core_register_dev_region() like everything else exported from
this module.

Suggested-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Link: https://lore.kernel.org/r/2-v2-1bd95d72f298+e0e-vfio_pci_priv_jgg@nvidia.com


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