qemu: Don't access /proc/bus/pci unless graphics pass-thru is enabled

qemu: Don't access /proc/bus/pci unless graphics pass-thru is enabled

A recent changeset introduced a bug whereby an initialization function
that reads /proc/bus/pci is called from graphics set-up functions even
if pass-through graphics are not enabled.  If qemu is run without
permission to this file, this causes qemu to fail during
initialization.

This patch re-works the functions so that the initialization happens
only if we actually need to do the pci host read or write.  It also
makes failures call abort().

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>