vdpa/mlx5: Move some definitions to a new header file

Move some definitions from mlx5_vnet.c to newly added header file
mlx5_vnet.h. We need these definitions for the following patches that
add debugfs tree to expose information vital for debug.

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>
Message-Id: <20221114131759.57883-7-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>