vdpa: merge functionally duplicated dev_features attributes

We can merge VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES with
VDPA_ATTR_DEV_FEATURES which is functionally equivalent.
While at it, tweak the comment in header file to make
user provioned device features distinguished from those
supported by the parent mgmtdev device: the former of
which can be inherited as a whole from the latter, or
can be a subset of the latter if explicitly specified.

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Message-Id: <1665422823-18364-1-git-send-email-si-wei.liu@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>