otautil: add support for unmounting entire volumes

When wiping /system, the partition isn't actually mounted at /system
or / - it's mounted at /mnt/system. This breaks 'format system' from
recovery if the partition has been mounted.

This patch adds an ensure_volume_unmounted function that finds all
mounts of a given device and unmounts them, meaning the device
can be safely formatted.

Change-Id: Id4f727f845308a89e865f1ba60dc284f5ebc66e1