Fixed umounting mountpoints at container stop.

Again fix for this feature. :)
Umounting didn't work in previous revision because there are following lines in 'mount' for containers:

/dev/mapper/blah on /var/lxc/root/blah type ext4 (rw,noatime,barrier=0)

and variable $root_path/$options{'contname'}/rootfs contains:

/var/lxc/root/blah/rootfs

Of course it would never be found in 'mount' output.
Also it's quite a bad idea to hardcode 'rootfs' dir rather than use option from config file.