Close #351: add more info to backup log

Close #351: add more info to backup log

Before patch:
INFO: exclude disk 'scsi1' (backup=no)
INFO: skip unused drive 'local:401/vm-401-disk-3.raw' (not included into backup)
INFO: skip unused drive 'local:401/vm-401-disk-1.raw' (not included into backup)

After patch applied:
INFO: include disk 'scsi0' local:401/vm-401-disk-4.qcow2
INFO: exclude disk 'scsi1' local:401/vm-401-disk-2.raw (backup=no)
INFO: include disk 'scsi2' pve4tank:vm-401-disk-1
INFO: skip unused drive 'local:401/vm-401-disk-3.raw' (not included into backup)
INFO: skip unused drive 'local:401/vm-401-disk-1.raw' (not included into backup)