Make sure a loop device exists before kpartx is called

If "kpartx -av" is called and there is no unused /dev/loop* device,
then kpartx fails. Calling "losetup -f" finds first unused device
and creates it if it doesn't exist.

Fixes bug #1195264

Change-Id: I5d59b519fd8e2a7380b71a788f9d3f2331b6567f