vdpa/mlx5: Report max device capabilities

vdpa/mlx5: Report max device capabilities

Configure max supported virtqueues and features on the management
device.
This info can be retrieved using:

$ 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

Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20220105114646.577224-12-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Si-Wei Liu<si-wei.liu@oracle.com>