vdpa: Support reading device features

vdpa: Support reading device features

When showing the available management devices, check if
VDPA_ATTR_DEV_SUPPORTED_FEATURES feature is available and print the
supported features for a management device.

Examples:
$ vdpa mgmtdev show
auxiliary/mlx5_core.sf.1:
  supported_classes net
  max_supported_vqs 257
  dev_features CSUM GUEST_CSUM MTU HOST_TSO4 HOST_TSO6 STATUS CTRL_VQ MQ \
               CTRL_MAC_ADDR VERSION_1 ACCESS_PLATFORM

$ vdpa -jp mgmtdev show
{
    "mgmtdev": {
        "auxiliary/mlx5_core.sf.1": {
            "supported_classes": [ "net" ],
            "max_supported_vqs": 257,
            "dev_features": [
"CSUM","GUEST_CSUM","MTU","HOST_TSO4","HOST_TSO6","STATUS","CTRL_VQ","MQ",\
"CTRL_MAC_ADDR","VERSION_1","ACCESS_PLATFORM" ]
        }
    }
}

Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eli Cohen <elic@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>