KVM: selftests: Use ARRAY_SIZE for array length

Use of macro ARRAY_SIZE to calculate array size minimizes
the redundant code and improves code reusability.

./tools/testing/selftests/kvm/x86_64/debug_regs.c:169:32-33: WARNING: Use ARRAY_SIZE.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=10847
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240913054315.130832-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: wb-zjp846396

tools/testing/selftests/kvm/x86_64/debug_regs.c

@@ -166,7 +166,7 @@ int main(void)
 	/* Test single step */
 	target_rip = CAST_TO_RIP(ss_start);
 	target_dr6 = 0xffff4ff0ULL;
-	for (i = 0; i < (sizeof(ss_size) / sizeof(ss_size[0])); i++) {
+	for (i = 0; i < ARRAY_SIZE(ss_size); i++) {
 		target_rip += ss_size[i];
 		memset(&debug, 0, sizeof(debug));
 		debug.control = KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_SINGLESTEP |