Additional check for block device

This will fix the installation if there is existing partitions.
The check is needed since the data is cached and wiping will be
done first to the actual disk and then to the partition. If such
partition does not exist then it will create a file instead.

Change-Id: I6f8f9388adb20b7d8496be38ad3676576f2eee6a
Signed-off-by: Janne Suominen <janne.suominen@nokia.com>