Fix string representation of VolumeBackupsRestore

Change string representation of VolumeBackupsRestore to use
volume_id rather than non-existent VolumeBackupsRestore id.

Change-Id: I7b585d0de9b3b0511e7fd1a70b214090a17dfa93
Closes-Bug: #1255488
Merge "Fix string representation of VolumeBackupsRestore"