qemu: assign PCI address to primary video card

qemu: assign PCI address to primary video card

When adding support for Q35 guests, the code to assign a PCI address
to the primary video card was moved into Q35 and i440fx(PIIX3)
specific functions, but no fallback was kept for other machine types
that might have a video card.

This patch remedies that by assigning a PCI address to the primary
video card if it does not have any kind of address.  In particular,
this fixes issues with pseries guests.

Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>
Signed-off-by: Laine Stump <laine@laine.org>