LRU unpin policy is an internal unpin policy which is triggered by
a timer. This policy can be used when there is no external balloon
unpin request.
BUG=b:188481989
TEST=Boot a VM with coiommu enabled + pass through devices.
Change-Id: Icb6e19073cb668fa954aec97e02be77f1b8f6a04
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3292937
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: David Stevens <stevensd@chromium.org>
