KVM: arm64: Add kunit test for ID register validation

Add kunit tests for functions that are used for validation of ID
registers and CONFIG_KVM_KUNIT_TEST option to enable the tests.

Since those tests only cover ID register validation so far, only
a few lines of change are needed in the default arm64.py to run
all of those tests as follows.
-----------------------------------------------------------------------
$ diff tools/testing/kunit/qemu_configs/arm64.py arm64_kvm_min.py
4a5,7
> CONFIG_VIRTUALIZATION=y
> CONFIG_KVM=y
> CONFIG_KVM_KUNIT_TEST=y
12c15
< 			   extra_qemu_params=['-machine virt', '-cpu cortex-a57'])

arch/arm64/kvm/Kconfig

@@ -54,4 +54,15 @@ config NVHE_EL2_DEBUG
 
 	  If unsure, say N.
 
+config KVM_KUNIT_TEST
+	bool "KUnit tests for KVM on ARM64 processors" if !KUNIT_ALL_TESTS
+	depends on KVM && KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  Say Y here to enable KUnit tests for the KVM on ARM64.
+	  Only useful for KVM/ARM development and are not for inclusion into
+	  a production build.
+
+	  If unsure, say N.
+
 endif # VIRTUALIZATION

arch/arm64/kvm/sys_regs.c

@@ -3630,3 +3630,7 @@ void kvm_sys_reg_table_init(void)
 
 	id_reg_info_init_all();
 }
+
+#if IS_ENABLED(CONFIG_KVM_KUNIT_TEST)
+#include "sys_regs_test.c"
+#endif