virtio: introduce extended features

virtio: introduce extended features

The virtio specifications allows for up to 128 bits for the
device features. Soon we are going to use some of the 'extended'
bits features (above 64) for the virtio_net driver.

Introduce extended features as a fixed size array of u64. To minimize
the diffstat allows legacy driver to access the low 64 bits via a
transparent union.

Introduce an extended get_extended_features configuration callback
that devices supporting the extended features range must implement in
place of the traditional one.

Note that legacy and transport features don't need any change, as
they are always in the low 64 bit range.

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>