vdpa_sim: add supported_features field in vdpasim_dev_attr

vdpa_sim: add supported_features field in vdpasim_dev_attr

Introduce a new VDPASIM_FEATURES macro with the generic features
supported by the vDPA simulator, and VDPASIM_NET_FEATURES macro with
vDPA-net features.

Add 'supported_features' field in vdpasim_dev_attr, to allow devices
to specify their features.

Co-developed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-9-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>