virtio-gpu-pci: add virtio pci support

This patch adds virtio-gpu-pci, which is the pci proxy for the virtio
gpu device.  With this patch in place virtio-gpu is functional.  You
need a linux guest with a virtio-gpu driver though, and output will
appear pretty late in boot, once the kernel initialized drm and fbcon.

Written by Dave Airlie and Gerd Hoffmann.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
