This led to VID being marked as available during unmount, and another drive could try an mount the same tape.
Added various logs.

The following sequence was observed in tests (with a sleep(3) at the end of MigrationReportPacker::ReportDriveStatus::execute() ):
 [eric@localhost ~]$ date; kubectl -n cta exec ctacli -- cta dr ls VDSTK11
Thu Jun 22 11:19:10 CEST 2017
    drive   host  library  mountType    status  desiredUp  forceDown     vid
  VDSTK11  tpsrv  VLSTK10    Archive  Mounting         UP             V01001
[eric@localhost ~]$ date; kubectl -n cta exec ctacli -- cta dr ls VDSTK11
Thu Jun 22 11:19:12 CEST 2017
    drive   host  library  mountType       status  desiredUp  forceDown     vid
  VDSTK11  tpsrv  VLSTK10    Archive  Transfering         UP             V01001
[eric@localhost ~]$ date; kubectl -n cta exec ctacli -- cta dr ls VDSTK11
Thu Jun 22 11:19:16 CEST 2017
    drive   host  library  mountType      status  desiredUp  forceDown     vid
  VDSTK11  tpsrv  VLSTK10    Archive  CleaningUp         UP             V01001
[eric@localhost ~]$ date; kubectl -n cta exec ctacli -- cta dr ls VDSTK11
Thu Jun 22 11:19:18 CEST 2017
    drive   host  library  mountType     status  desiredUp  forceDown     vid
  VDSTK11  tpsrv  VLSTK10    Archive  Unloading         UP             V01001
[eric@localhost ~]$ date; kubectl -n cta exec ctacli -- cta dr ls VDSTK11
Thu Jun 22 11:19:22 CEST 2017
    drive   host  library  mountType      status  desiredUp  forceDown     vid
  VDSTK11  tpsrv  VLSTK10    Archive  Unmounting         UP             V01001
[eric@localhost ~]$ date; kubectl -n cta exec ctacli -- cta dr ls VDSTK11
Thu Jun 22 11:19:25 CEST 2017
    drive   host  library  mountType  status  desiredUp  forceDown  vid
  VDSTK11  tpsrv  VLSTK10    NoMount      Up         UP               -