The typical case where we had a problem is with such a filesystem
definition as created by virt-sandbox-service:

    <filesystem type='bind' accessmode='passthrough'>
      <source dir='/var/lib/libvirt/filesystems/mysshd/var'/>
      <target dir='/var'/>
    </filesystem>

In this case, we don't want to unmount the /var subtree or we may
loose the access to the source folder.