vga: rework bar detection, add virtio-vga

Replace the heuristic which checks the bar 0 attributes to
figure whenever bar 0 or bar 1 has the linear framebuffer
with a vendor id check.

Add support for virtio-vga which uses bar 2 for the linear
framebuffer.

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