KVM: selftests: arm64: Initialise default guest mode at test startup time

As we are going to add support for a variable default mode on arm64,
let's make sure it is setup first by using a constructor that gets
called before the actual test runs.

Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Link: https://lore.kernel.org/r/20211227124809.1335409-2-maz@kernel.org