wrap LoopDevice in a struct that implements drop
wrap LoopDevice in a struct that implements drop

When the struct goes out of scope, the loop device is detached.
If test loopdevs with Stratis data are not cleaned up, subseqent
tests may fail.

Signed-off-by: Todd Gill <7afbb5ba98932b8ffb9cc9e6675c4792db142529@redhat.com>

trgill/stratisd,mulkieran/stratisd,trgill/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,mulkieran/stratisd,stratis-storage/stratisd