KVM: selftests: Allow dumping per-vcpu info for uffd threads

There's one PER_VCPU_DEBUG in per-vcpu uffd threads but it's never hit.

Trigger that when quit in normal ways (kick pollfd[1]), meanwhile fix the
number of nanosec calculation.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20230427201112.2164776-3-peterx@redhat.com


Signed-off-by: Sean Christopherson <seanjc@google.com>