Add a shared targets flag to volumes as well as a unique backend id

  Currently when os-brick recieves a detach operation it attempts to inspect the
  system and determine if there are multiple volumes attached to the host sharing
  the same target. Most of the time this works, however it tends to be a bit
  racey and sometimes attempting bus scans is not only ineffecient but
  unreliable.

  The other problem is that currently there is no indication for os-brick to know
  if it's dealing with a device that utilizes shared targets or not, so we go
  through this scanning routine even if we don't necessarily need to.

  The process of removing targets would be much more robust if we actually knew
  ahead of time if the volume was hosted on a device that utilizes shared
  targets, in which case we could do things more efficiently including using
  locks on a compute node around attach/detach operations for a specific backend.