qemu: make virtio-blk PCI compliant by default

commit bf011293faaa7f87e4de83185931e7411b794128 made virtio-blk-pci not
PCI-compliant, since it makes region 0 (which is an i/o region)
size > 256, and, since PCI 2.1, i/o regions are limited to 256 bytes size.

When the ATA serial number feature is off, which is the default,
make the device spec compliant again, by making region 0 smaller.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reported-by: Vadim Rozenfeld <vrozenfe@redhat.com>
Tested-by: Vadim Rozenfeld <vrozenfe@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
