vdpa/mlx5: optimize notification events

vdpa/mlx5: optimize notification events

When the virtio guest driver doesn't work with poll mode, the driver
creates event mechanism in order to schedule completion notifications
for each virtq burst traffic.

When traffic comes to a virtq, a CQE will be added to the virtq CQ by
the FW.
The driver requests interrupt for the next CQE index, and when interrupt
is triggered, the driver polls the CQ and notifies the guest by virtq
callfd writing.

According to the described method, the interrupts will be triggered for
each burst of traffic. The burst size depends on interrupt latency.

Interrupts management takes a lot of CPU cycles and using it for each
traffic burst takes big portion of CPU capacity.

When traffic is on, using timer for CQ poll scheduling instead of
interrupts saves a lot of CPU cycles.

Move CQ poll scheduling to be done by timer in case of running traffic.
Request interrupts only when traffic is off.

The timer scheduling management is done by a new dedicated thread uses
a usleep command.

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