Disable CPU dirty region tracking on Xen + Arm64 where xen migration is not supported.
cpu_physical_memory_set_dirty_range
performs no checks on whether xen is active on the aarch64 target. Dirty logging bitmaps is used for VM migration which in Xen is x86 only.
There needs to be a new check added, or even better, characterize XenIOState
which is shared by all HVM guests with an immutable migratable
property.