net/mlx5: Provide external API for allocating vectors

Provide external API to be used by other drivers relying on mlx5_core,
for allocating MSIX vectors. An example for such a driver would be
mlx5_vdpa.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>