option to sparsify raw images

This would particularly be a need for the BitCurator/forensics crowd: raw disk images of system drives are likely to be very large and unwieldy to move around. It might be a handy feature to build into Qiwi the ability to create a sparsified copy of the raw disk image (remove empty/unused space in the image from the host device's storage), as a way of creating an "access copy" of a system disk (that can then be moved around and run more easily with QEMU, EaaSI, wherever)

this might help encourage uptake in general of emulation as an assessment tool for archival disk images