Plugin lxc: verify the availability of "lxc-ls"

Previously "autoconf" only verified, that /var/lib/lxc exists.  But this
could be just a remainder of a formerly installed lxc package.

plugins/lxc/lxc-multigraph

@@ -183,6 +183,8 @@ do_autoconf() {
       echo "no (/proc/net/dev cannot be read)"
    elif [ ! -e "$lxcpath" ]; then
       echo "no ($lxcpath is not present)"
+   elif [ -z "$(which lxc-ls)" ]; then
+      echo "no ('lxc-ls' is not available in PATH)"
    else
       echo yes
    fi