ANDROID: KVM: arm64: Flag pages needing poisoning in hyp_vmemmap

During teardown, we currently walk the guest stage-2 page-table and
annotate all of its pages as 'pending poisoning' in the host stage-2.
Sadly, this requires a host stage-2 walk for every guest page, which is
rather inefficient and can lead to a long non-preemptible amount of time
spent at EL2. This gets particularly bad with IOMMUs as, in its current
form, the host stage-2 annotation triggers IOMMU updates.

To avoid the host stage-2 walks, let's annotate the pages pending
poisoning using a flag in the hyp_vmemmap instead.

Bug: 219180169
Signed-off-by: Quentin Perret <qperret@google.com>
Change-Id: I8894bd8e0b10ea8817763479412b540c0291e8f5