net/mlx5: support new representor naming format

net/mlx5: support new representor naming format

Kernel update [1] introduce new format of representors names.
This patch implements RFC [2], updating MLX5 PMD to support the new
format, while maintaining support of the existing format.

[1] https://github.com/torvalds/linux/commit/c12ecc2
[2] http://mails.dpdk.org/archives/dev/2019-March/125676.html

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>