vdpa/mlx5: add multi-thread management for configuration

vdpa/mlx5: add multi-thread management for configuration

The LM process includes a lot of objects creations and
destructions in the source and the destination servers.
As much as LM time increases, the packet drop of the VM increases.
To improve LM time need to parallel the configurations for mlx5 FW.
Add internal multi-thread management in the driver for it.

A new devarg defines the number of threads and their CPU.
The management is shared between all the devices of the driver.
Since the event_core also affects the datapath events thread,
reduce the priority of the datapath event thread to
allow fast configuration of the devices doing the LM.

Signed-off-by: Li Zhang <lizh@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>