KVM: arm64: selftests: Use FIELD_GET() to extract ID register fields

Use FIELD_GET() macro to extract ID register fields for existing
aarch64 selftests code. No functional change intended.

Signed-off-by: Reiji Watanabe <reijiw@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221020054202.2119018-2-reijiw@google.com