virtio-mmio: implement modern (v2) personality (virtio-1)

virtio-mmio: implement modern (v2) personality (virtio-1)

Implement the modern (v2) personality, according to the VirtIO 1.0
specification.

Support for v2 among guests is not as widespread as it'd be
desirable. While the Linux driver has had it for a while, support is
missing, at least, from Tianocore EDK II, NetBSD and FreeBSD.

For this reason, the v2 personality is disabled, keeping the legacy
behavior as default. Machine types willing to use v2, can enable it
using MachineClass's compat_props.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Message-Id: <20190913120559.40835-1-slp@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>