[ena] Allocate an unused Asynchronous Event Notification Queue (AENQ)

[ena] Allocate an unused Asynchronous Event Notification Queue (AENQ)

We currently don't allocate an Asynchronous Event Notification Queue
(AENQ) because we don't actually care about any of the events that may
come in.

The ENA firmware found on Graviton instances requires the AENQ to
exist, otherwise all admin queue commands will fail.

Fix by allocating an AENQ and disabling all events (so that we do not
need to include code to acknowledge any events that may arrive).

Signed-off-by: Alexander Graf <graf@amazon.com>