lguest: implement virtio-PCI MMIO accesses.

For each device, We need to include the vendor capabilities to demark
where virtio common, notification and ISR regions are (we put them
all in BAR0).

We need to handle the switching of the virtqueues using the accessors.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --cc tools/lguest/lguest.c
Simple merge