Add test for virsh maxvcpus

Add a test for virsh maxvcpus

1. add virsh maxvcpus funtion in virttest.virsh
2. add virsh_maxvcpus.py
3. add virsh_maxvcpus.cfg

Updated to v3 now

v2 ~ v3:
* Move test back to host dir
* Update connect_uri to use utils_conn
* Adjust cfg to include new unexpected option