implement copy-on-read for disk images

[Marc Schütz]

There is already support for read-only images with copy-on-write semantics. In some cases, where the disk image resides on a NAS connected over a relatively slow network link (e.g. Samba, NFS), it is desirable to have copy-on-read behaviour too:

Whenever a block from the read-only image is read, it is stored in the differential image (which the user can place on a local partition). The second and following accesses to the same block (e.g. when the VM is rebooted) will then be considerably faster.

This way, fast image access can be provided without explicitly copying the entire image to the local machine.