basic: detect_vm_cpuid: use gcc's __get_cpuid() function (#7758)

The __get_cpuid() function includes a safety check to ensure that
executing the cpuid instruction is valid/safe.

This method also works with clang.

https://lists.freedesktop.org/archives/systemd-devel/2017-December/040054.html
(cherry picked from commit d31b0033)