virttest.utils_misc:fix 'unknown' CPU for IBM Power architecture

Currently, virt-tests support amd and intel's X86 cpu type,
IBM Power cpus will be detected as 'unknown'.

To solve the problem, use platform.machine, so when the
arch is ppc64, we can return appropriate power CPU models.
Signed-off-by: NMike Qiu <qiudayu@linux.vnet.ibm.com>