virtio-blk: set config size depending on the features enabled

Starting from DISABLE and WRITE_ZEROES features, we use an array of
VirtIOFeature (as virtio-net) to properly set the config size
depending on the features enabled.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20190221103314.58500-6-sgarzare@redhat.com
Message-Id: <20190221103314.58500-6-sgarzare@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>