Implement the VIRTIO 1.0 virtio-pci interface.  The main change here is
that the register layout is no longer a fixed layout in BAR 0.  Instead
we have to iterate of PCI Capabilities to find descriptions of where
various registers are located.  The vring registers are also more
fine-grained, allowing for more flexible vring layouts, but we don't
take advantage of that.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Message-Id: <20191023100425.12168-17-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>