logical: Need to overwrite/clear more than just first 512 bytes

logical: Need to overwrite/clear more than just first 512 bytes

https://bugzilla.redhat.com/show_bug.cgi?id=1430679

As it turns out some file headers (e.g. ext4) may be larger/longer than
the 512 bytes of zeros being written prior to a pvcreate, so let's write
out 2048 bytes similar to how the pvcreate sources would peek at the first
4 sectors of the device.

Make sure there is at enough bytes on the device to clear before doing
doing the clear - just to be sure.