qemu vfio-pci: Add support for VGA MMIO and I/O port access

qemu vfio-pci: Add support for VGA MMIO and I/O port access

Most VGA cards need some kind of quirk to fully operate since they
hide backdoors to get to other registers outside of PCI config space
within the registers, but this provides the base infrastructure.  If
we could identity map PCI resources for assigned devices we would need
a lot fewer quirks.

To enable this, use a kernel side vfio-pci driver that incorporates
VGA support (v3.9), and use the -vga none option and add the x-vga=on
option for the vfio-pci device.  The "x-" denotes this as an
experimental feature.  You may also need to use a cached copy of the
VGA BIOS for your device, passing it to vfio-pci using the romfile=
option.

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