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

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Merge pull request #622 from yangdongsheng/utils_libvirtd

utils_libvirtd: if libvirt is not availible in host, No raising Error, but logging.warning.