1. A method of backing up a virtual machine, the method comprising: receiving an instruction to track changes to a virtual hard disk; generating a token in response to receiving the instruction to track changes to the virtual hard disk; intercepting, by a first driver under the control of a first hypervisor which executes on a first client computing device, a first write operation generated by a virtual machine (VM) to store data in a first sector of the virtual hard disk, wherein the VM is executed by the first hypervisor, wherein the VM comprises a virtual hard disk file and a change block bitmap file, and wherein the virtual hard disk file stores data associated with the virtual hard disk; with the first client computing device, accessing the change block bitmap file by providing the token to the first driver and modifying a first entry in the change block bitmap file to indicate that data in the first sector has changed; migrating the VM to a second hypervisor which executes on a second client computing device, such that the VM is executed by the second hypervisor following said migrating; intercepting by a second driver under the control of the second hypervisor, a second write operation generated by the VM to store data in a second sector of the virtual hard disk; and with the second client computing device, accessing the change block bitmap file and modifying a second entry in the change block bitmap file to indicate that data in the second sector has changed, receiving an instruction to begin an incremental backup of the VM, and as part of the incremental backup of the VM, reviewing the first and second entries in the change block bitmap file to identify the first and second sectors as sectors to access in gathering data for use in performance of the incremental backup.