aarch64 support for software-managed dirty page tracking

[wjhun]

While the cpu type specified to qemu for the virt platform is armv8.1-a, a read of ID_AA64MMFR1_EL1 does not indicate that hardware management of dirty pages is available (e.g. HD and HA bits are zero). If we can't depend on this feature, we'll need to set shared pages to read-only and track dirty state via a protection exception.