pci: fix corrupted pci conf index register by unaligned write

Commit d0ed8076cbdc261 converted the PCI config access to the memory
API, but also inadvertantly changed it to accept unaligned writes,
and corrupt the index register in the process.  This causes a regression
booting NetBSD.

Fix by ignoring unaligned or non-dword writes.

https://bugs.launchpad.net/qemu/+bug/897771

Reported-by: Andreas Gustafsson <gson@gson.org>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
