After the P2V migration disk type of the resulted VM is split:
# prl_disk_tool convert -i --hdd /var/parallels/testDiskMigr.pvm/testDiskMigr.hdd/ Disk information: Type: expanding, split
This issue was recognized as a bug with internal ID #PSBM-28057.
The issue is fixed in Virtuozzo 6 update 8. The virtual machines with split disks should be merged manually:
# prl_disk_tool convert --hdd /var/parallels/testDiskMigr.pvm/testDiskMigr.hdd/ --merge
NOTE: VM should be stopped before you start merging the disk