ANDROID: KVM: arm64: iommu: Optimize snapshot_host_stage2

Currently the generic IOMMU code lets the driver initialize its PT and
then invokes callbacks to set the permissions across the entire PA
range. Optimize this by making it a requirement on the driver to
initialize its PTs to all memory owned by the host. snapshot_host_stage2
then only calls the driver's callback for memory regions not owned by
the host.

Bug: 190463801
Bug: 218012133
Signed-off-by: David Brazdil <dbrazdil@google.com>
Change-Id: I51ff38cb4f4e28e19903af942776b401504c363e