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

tools/testing/selftests/kvm/lib/aarch64/processor.c

@@ -432,3 +432,12 @@ uint32_t guest_get_vcpuid(void)
 {
 	return read_sysreg(tpidr_el1);
 }
+
+/*
+ * arm64 doesn't have a true default mode, so start by computing the
+ * available IPA space and page sizes early.
+ */
+void __attribute__((constructor)) init_guest_modes(void)
+{
+       guest_modes_append_default();
+}