baremetal: Fix misuse of "instance" parameter of attach/detach_volume

Some parts of baremetal driver treat "instance" parameter as if it is
the name of an instance, but actually it is the instance itself (dict).

Closes-Bug: #1221244
Change-Id: I01a296a6ac0bbf7c5fec3434ddf1197828b849b6
Merge "baremetal: Fix misuse of "instance" parameter of attach/detach_volume"