vdpa/mlx5: control completion queue event mode

vdpa/mlx5: control completion queue event mode

The CQ polling is necessary in order to manage guest notifications when
the guest doesn't work with poll mode (callfd != -1).

The CQ polling scheduling method can affect the host CPU utilization and
the traffic bandwidth.

Define 3 modes to control the CQ polling scheduling:

1. A timer thread which automatically adjusts its delays to the coming
   traffic rate.
2. A timer thread with fixed delay time.
3. Interrupts: Each CQE burst arms the CQ in order to get an interrupt
   event in the next traffic burst.

When traffic becomes off, mode 3 is taken automatically.

The interrupt management takes a lot of CPU cycles but forward traffic
event to the guest very fast.

Timer thread save the interrupt overhead but may add delay for the guest
notification.

Add device arguments to control on the mode.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>