virtio-pci-modern: introduce helpers for setting and getting features

This patch introduces helpers for setting and getting features.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210104065503.199631-8-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>