virttest.utils_misc: Add cpu model Opteron_G5 to get_host_cpu_models

Add the new AMD cpu model to the the cpu_model list.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #654 from ypu/cpumodel

virttest.utils_misc: Add cpu model Opteron_G5 to get_host_cpu_models