Lightly tested against qemu.
One thing *not* implemented here is separate mappings
for descriptor/avail/used rings. That's nice to have,
will be done later after we have core support.
This also exposes the PCI layout to userspace, and
adds macros for PCI layout offsets:
QEMU wants it, so why not? Trust, but verify.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
