CA-137238: Revert "CA-90424: Does not migrate VDI if it is already migrating."

The added assert meant that we couldn't make use of the queuing
functionality of VM.with_vm_operation. After the change which this
commit reverts, if one of a VM's disks was undergoing a VDI.pool_migrate
then attempting to VDI.pool_migrate another of the VM's disks would fail
immediately with OTHER_OPERATION_IN_PROGRESS.

After reverting this commit, the second VDI.pool_migrate gets queued up
behind the first.

This reverts commit 8598ea5b2dcda479a0c01ee081a3f7f999091087.