virttest.virsh: Add virsh maxvcpus function

Signed-off-by: Wayne Sun <gsun@redhat.com>
libvirt.test: Add virsh maxvcpus test

Add maxvcpus case under host dir, it supports local and remote
ssh connection.

The sample cfg offer test on local qemu connection and remote
qemu+ssh connection, with both supported and unexpected options.

The sample cfg use local host as remote server, remote host ip
and password could be provided in cfg for test.

v2 ~ v3:
* Move test back to host dir
* Update connect_uri to use utils_conn
* Adjust cfg to include new unexpected option

Signed-off-by: Wayne Sun <gsun@redhat.com>
libvirt: Fix more docstrings

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>