net/mlx5: DR, Add support for isolate_vl_tc QP

When using SW steering, rule insertion rate depends on the RDMA RC QP
performance used for writing to the ICM. During stress this QP is competing
on the HW resources with all the other QPs that are used to send data.
To protect SW steering QP's performance in such cases, we set this QP to
use isolated VL. The VL number is reserved by FW and is not exposed to the
driver.
Support for this QP on isolated VL exists only when both force-loopback and
isolate_vl_tc capabilities are set.

Signed-off-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>