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: Nyangdongsheng <yangds.fnst@cn.fujitsu.com>