utils_libvirtd: if libvirt is not availible in host, No error, but a warning.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
virttest.aexpect: Fix old class Spawn definition.

The base class Spawn for ShellSession was defined in old way(class Spawn).
Then we can not use super() for inherited classes:
TypeError: super() argument 1 must be type, not classobj

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #624 from yumingfei/aexpect_spawn

Aexpect spawn's definition
Merge pull request #622 from yangdongsheng/utils_libvirtd

utils_libvirtd: if libvirt is not availible in host, No raising Error, but logging.warning.
virttest.env_process: Remove unused import libvirt_vm

Signed-off-by: Yiqiao Pu <ypu@redhat.com>