Slow BAR access path is used when VFIO fails to mmap() BAR.
Since this is just a transport between the guest and a device, there is
no need to do endianness swapping.

This changes BARs to use native endianness. Since non-ROM BARs were
doing byte swapping, we need to remove it so does the patch.
As the result, this eliminates cancelling byte swaps and there is
no change in behavior for non-ROM BARs.

ROM BARs were declared little endian too but byte swapping was not
implemented for them so they never actually worked on big endian systems
as there was no cancelling byte swap. This fixes endiannes for ROM BARs
by declaring them native endian and only fixing access sizes as it is
done for non-ROM BARs.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>