virttest.utils_conn: add auto_recover into __init__().

B/c the errors in __del__() will be ignored, then we want
user call conn_recover() manually our of __del__().

* rename conn_finish to conn_recover.
* add auto_recover into __init__.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
libvirt-tests: replace /var/run/libvirt/lxc with /usr/libvirtexec/libvirt_lxc.

Path /var/run/libvirt/lxc means there is a running lxc guest, but not the lxc
driver is available.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>