KVM: arm64: Use read-only helper for reading VM ID registers

KVM: arm64: Use read-only helper for reading VM ID registers

IDREG() expands to the storage of a particular ID reg, which can be
useful for handling both reads and writes. However, outside of a select
few situations, the ID registers should be considered read only.

Replace current readers with a new macro that expands to the value of
the field rather than the field itself.

Reviewed-by: Sebastian Ott <sebott@redhat.com>
Link: https://lore.kernel.org/r/20240619174036.483943-4-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>