Why do imported VMs with multiple disks fail to start in RHEVM 3.1 with NullPointerException in engine.log?

Issue

• VMs imported from RHEV2.2 into RHEV3.1
• VMs with 1 disk are starting fine, but VMs with 2 disks are failing to start
• Observing the following error in /var/log/ovirt-engine/engine.log:

2012-12-13 00:45:30,128 ERROR [org.ovirt.engine.core.vdsbroker.CreateVmVDSCommand] (pool-4-thread-50) [59d76e39] Error in excuting CreateVmVDSCommand: java.lang.NullPointerException
        at org.ovirt.engine.core.dal.comparators.DiskImageByDiskAliasComparator.compare(DiskImageByDiskAliasComparator.java:11) [engine-dal.jar:]
        at org.ovirt.engine.core.dal.comparators.DiskImageByDiskAliasComparator.compare(DiskImageByDiskAliasComparator.java:7) [engine-dal.jar:]
        at java.util.TimSort.countRunAndMakeAscending(TimSort.java:324) [rt.jar:1.7.0_09-icedtea]
        at java.util.TimSort.sort(TimSort.java:189) [rt.jar:1.7.0_09-icedtea]
        at java.util.TimSort.sort(TimSort.java:173) [rt.jar:1.7.0_09-icedtea]
        at java.util.Arrays.sort(Arrays.java:659) [rt.jar:1.7.0_09-icedtea]
        at java.util.Collections.sort(Collections.java:217) [rt.jar:1.7.0_09-icedtea]
        at org.ovirt.engine.core.vdsbroker.vdsbroker.VmInfoBuilderBase.getSortedDisks(VmInfoBuilderBase.java:189) [engine-vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.vdsbroker.VmInfoBuilder.buildVmDrives(VmInfoBuilder.java:225) [engine-vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.vdsbroker.CreateVDSCommand.buildVmData(CreateVDSCommand.java:64) [engine-vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.vdsbroker.CreateVDSCommand.ExecuteVdsBrokerCommand(CreateVDSCommand.java:24) [engine-vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.vdsbroker.VdsBrokerCommand.ExecuteVDSCommand(VdsBrokerCommand.java:91) [engine-vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.VDSCommandBase.ExecuteCommand(VDSCommandBase.java:63) [engine-vdsbroker.jar:]
        at org.ovirt.engine.core.dal.VdcCommandBase.Execute(VdcCommandBase.java:41) [engine-dal.jar:]
        at org.ovirt.engine.core.vdsbroker.CreateVmVDSCommand.ExecuteVdsIdCommand(CreateVmVDSCommand.java:65) [engine-vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.VdsIdVDSCommandBase.ExecuteVDSCommand(VdsIdVDSCommandBase.java:49) [engine-vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.VDSCommandBase.ExecuteCommand(VDSCommandBase.java:63) [engine-vdsbroker.jar:]
        at org.ovirt.engine.core.dal.VdcCommandBase.Execute(VdcCommandBase.java:41) [engine-dal.jar:]
        at org.ovirt.engine.core.vdsbroker.ResourceManager.runVdsCommand(ResourceManager.java:388) [engine-vdsbroker.jar:]
        at org.ovirt.engine.core.bll.VDSBrokerFrontendImpl.RunVdsCommand(VDSBrokerFrontendImpl.java:33) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.VDSBrokerFrontendImpl.RunAsyncVdsCommand(VDSBrokerFrontendImpl.java:48) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.RunVmCommand.CreateVm(RunVmCommand.java:407) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.RunVmCommand.RunVm(RunVmCommand.java:201) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.RunVmCommand.ExecuteVmCommand(RunVmCommand.java:255) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.VmCommand.executeCommand(VmCommand.java:78) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.CommandBase.executeWithoutTransaction(CommandBase.java:876) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.CommandBase.executeActionInTransactionScope(CommandBase.java:973) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.CommandBase.runInTransaction(CommandBase.java:1372) [engine-bll.jar:]
        at org.ovirt.engine.core.utils.transaction.TransactionSupport.executeInSuppressed(TransactionSupport.java:168) [engine-utils.jar:]
        at org.ovirt.engine.core.utils.transaction.TransactionSupport.executeInScope(TransactionSupport.java:107) [engine-utils.jar:]