CA-140684: Handle cancellation of VM migration properly

Suggested-by: Yunlei Ding <yunlei.ding@citrix.com>
Signed-off-by: Euan Harris <euan.harris@citrix.com>
Signed-off-by: Rob Hoes <rob.hoes@citrix.com>

Conflicts:
	ocaml/xapi/xapi_vm_migrate.ml
Merge pull request #1898 from robhoes/cancel

CA-140684: Handle cancellation of VM migration properly