Add support for additional regions with indexes started after the
already defined fixed regions.  Device specific code can register
these regions with the new vfio_pci_register_dev_region() function.
The ops structure per region currently only includes read/write
access and a release function, allowing automatic cleanup when the
device is closed.  mmap support is only missing here because it's
not needed by the first user queued for this support.
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>