vfio-pci: Fix Nvidia MSI ACK through 0x88000 quirk

When MSI is enabled on Nvidia GeForce cards the driver seems to
acknowledge the interrupt by writing a 0xff byte to the MSI capability
ID register using the PCI config space mirror at offset 0x88000 from
BAR0.  Without this, the device will only fire a single interrupt.
VFIO handles the PCI capability ID/next registers as virtual w/o write
support, so any write through config space is currently dropped.  Add
a check for this and allow the write through the BAR window.  The
registers are read-only anyway.

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