Adapt "nova rescue" code to support Virtuozzo ploop disks.
In rescue mode we use static mount point /mnt/rescue for original filesystem.
As far as ploop disks are in fact directories
we use rmtree function to delete rescue disks in unrescue procedure.
Implements: blueprint virtuozzo-rescue-support
Change-Id: I07b9e3a8b726635294395b0292fc3592a1b0f918