vdpa: Support reporting max device capabilities

vdpa: Support reporting max device capabilities

Add max_supported_vqs and supported_features fields to struct
vdpa_mgmt_dev. Upstream drivers need to feel these values according to
the device capabilities.

These values are reported back in a netlink message when showing management
devices.

Examples:

$ 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 -j 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"]}}}

$ 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"]
        }
    }
}

Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20220105114646.577224-11-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Si-Wei Liu<si-wei.liu@oracle.com>