vfio/pci: Virtualize Maximum Payload Size

With virtual PCI-Express chipsets, we now see userspace/guest drivers
trying to match the physical MPS setting to a virtual downstream port.
Of course a lone physical device surrounded by virtual interconnects
cannot make a correct decision for a proper MPS setting.  Instead,
let's virtualize the MPS control register so that writes through to
hardware are disallowed.  Userspace drivers like QEMU assume they can
write anything to the device and we'll filter out anything dangerous.
Since mismatched MPS can lead to AER and other faults, let's add it
to the kernel side rather than relying on userspace virtualization to
handle it.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>