vzctl can't destroy ve_private if ploop is mounted

For some reason, sometimes ploop is left semi-mounted (i.e. ploop device is present, but not mounted), and in such cases vzctl fails to destroy CT private:

# vzctl destroy 4956949
Destroying container private area: /vz/private/4956949
Warning: failed to remove /vz/root/4956949: Device or resource busy
Container private area was destroyed

I guess we need to add an additional check that ploop is not mounted before proceeding with private area removal.