vfio-pci: Cleanup BAR access

We can actually handle MMIO and I/O port from the same access function
since PCI already does abstraction of this.  The ROM BAR only requires
a minor difference, so it gets included too.  vfio_pci_config_readwrite
gets renamed for consistency.

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