KVM: arm64: Handle ID register reads using the VM-wide values

Everything is in place now to use the generic ID register
infrastructure. Use the VM-wide values to service ID register reads.
The ID registers are invariant after the VM has started, so there is no
need for locking in that case. This is rather desirable for VM live
migration, as the needless lock contention could prolong the VM blackout
period.

Link: https://lore.kernel.org/r/20230609190054.1542113-11-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>