Erase file system signatures before creating a partition table (#743181)

When writing "loop" partition table over the top of some whole disk
device file system types GParted continued to show those whole disk
device file systems rather than the virtual unknown partition from the
"loop" partition table.

This affected btrfs, jfs, reiser4 and reiserfs.  It occurred because of
several factors:
1) Libparted only zeroed the first and last 9.5 KiB (assuming 512 byte
   sectors) of the device before writing a new partition table.  See
   ped_disk_clobber().
2) These file systems have their super blocks and therefore signatures
   after the first 9.5 KiB.
3) Whole disk device file system detection is performed using blkid
   before checking for a libparted "loop" partition table.  See
   GParted_Core::set_devices_thread().

Ref:
libparted 3.2: disk.c:ped_disk_clobber()
http://git.savannah.gnu.org/cgit/parted.git/tree/libparted/disk.c?id=v3.2#n302

Fix by always erasing any possible file system signatures on the device
before creating a new "loop" partition table.

NOTE:
This is typically taking up to 0.5 seconds in my testing on a 5400 RPM
hard drive, during which time the GParted UI is hung and the create
partition table dialog shows the apply button pressed but no other
progress indication.

Bug 743181 - Add unpartitioned drive read-write support