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>