KVM: Explicitly allocate/setup irqfd cleanup as per-CPU workqueue

KVM: Explicitly allocate/setup irqfd cleanup as per-CPU workqueue

Explicitly request the use of per-CPU queues for the irqfd cleanup
workqueue in preparation for changing the default behavior of
alloc_workqueue() from per-CPU to unbound, which will in turn allow for
the removal of WQ_UNBOUND.  See commit 930c2ea566af ("workqueue: Add new
WQ_PERCPU flag") for details.

No functional change intended.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Link: https://lore.kernel.org/r/20250905091139.110677-2-marco.crivellari@suse.com
[sean: rewrite changelog to tailor it to the KVM]
Signed-off-by: Sean Christopherson <seanjc@google.com>