vhost-user-blk: make 'config_wce' part of 'host_features'

vhost-user-blk: make 'config_wce' part of 'host_features'

No reason to have this be a separate field. This also makes it more akin
to what the virtio-blk device does.

Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <20220906073111.353245-5-d-tatianin@yandex-team.ru>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>