#39 checking COW bakcing filesystems to avoid corruption

[Fermín Galán]

From https://lists.dit.upm.es/pipermail/vnuml-users/2006-November/000556.html:

"Before start a simulation involving COW filesystem, VNUML should check the backing filesystem of each COW filesystem haven't changed: in that case warm to the user and exit. The problem here is how to perform a quick check (md5sum and other hash would take a lot of time because the bakcing filesystem is several hundred MB long)."