fix bug #841: replace get_used_paths with is_volume_in_use

fix bug #841: replace get_used_paths with is_volume_in_use

get_used_paths returned a hash of used paths for all the
volumes in a VM's config, which is not enough to figure out
whether there are snapshots, as snapshots often have
different paths.  Eg. on ZFS it is not enough to check for
/dev/zvol/tank/vm-123-disk-1 because the snapshot's path is
/dev/zvol/tank/vm-123-disk-1@snap1 and thus we allowed
deleting the drive. Then when trying to delete the snapshot
later you get:
  zfs error: cannot open 'tank/vm-751-disk-1': dataset does not exist
and it refuses to delete the snapshot.

Since its only use was to check whether or not a drive is
still in use it is now renamed to is_volume_in_use and
beside checking paths now also checks volume-ids as those
should stay the same.