Make clearing signatures work with whole disk devices (#743181)

This enables Format to Cleared operation to succeed on whole disk device
file systems even when libparted doesn't recognise the file system.

(Turns out that making calibrate work in the previous commit happened to
make Format to Cleared operation succeed, but only if libparted
recognised the file system on the whole disk device).

Bug 743181 - Add unpartitioned drive read-write support