lscpu: improve hypervisor detection v2

Updated according to comments on util-linux@vger.kernel.org

changes since v1:
  * separate read_hypervisor_powerpc function:
    - fopen -> path_fopen
    - it works correct now for  SYSTEM_SNAPSHOT and not only ifdef __powerpc_
  * VMWARE_BDOOR:
    - cleanup ifdefs
    - attribute unused
    - must be only called if not SYSTEM_SNAPSHOT

Note VMWARE_BDOOR breaks travis build for (old) clang. To be fixed later via ifdef or just for travis.