Returns several useful information about Qcow2 file

This script can returns:
- the number of allocated clusters in a Qcow file.
- the number of newly allocated clusters in a Qcow file
  compared to a reference one (like the backing file).
- it can erase all data and set clusters as unallocated
- it can set/unset/get the hidden property.
  - for now it creates the custom header if not found but it can easily
    be changed. It also moves the backing file name because when a
    snapshot is created the hidden custom header is ignored so we need
    to make space to crate it again.

NOTE: This script is a POC to validate that all helpers from vhd util
are also doable with qcow file. It will be used in another file.

Signed-off-by: Guillaume <[email protected]>