qemu/pci: clarify pci config load routine

PCI load routine has to be called with size equal to 256 (otherwise it
will crash in weird ways).  So assert this, making code clearer.
Also avoid dynamically sized array on stack - good for portability.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
