preseed_create: osstest-erase-other-disks: workaround creating /dev/sdXD files

preseed_create: osstest-erase-other-disks: workaround creating /dev/sdXD files

Sometime, it can happen that the erase-other-disks script fails and
indicate that /dev/sda1 isn't a block device anymore. This happened
when /dev/sda was erased before /dev/sda1. So to avoid this, we will
zero the partitions of ${disk} first, and hopefully the error won't
happen again.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>