KVM: arm64: nv: Use accessors for modifying ID registers

KVM: arm64: nv: Use accessors for modifying ID registers

In the interest of abstracting away the underlying storage of feature
ID registers, rework the nested code to go through the accessors instead
of directly iterating the id_regs array.

This means we now lose the property that ID registers unknown to the
nested code get zeroed, but we really ought to be handling those
explicitly going forward.

Link: https://lore.kernel.org/r/20240619174036.483943-6-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>