pci: fix corrupted pci conf index register by unaligned write

Commit d0ed8076 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/897771Reported-by: NAndreas Gustafsson <gson@gson.org>
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/pci_host.c

@@ -101,6 +101,9 @@ static void pci_host_config_write(void *opaque, target_phys_addr_t addr,
 
     PCI_DPRINTF("%s addr " TARGET_FMT_plx " len %d val %"PRIx64"\n",
                 __func__, addr, len, val);
+    if (addr != 0 || len != 4) {
+        return;
+    }
     s->config_reg = val;
 }