We forgot to add virDomainUndefineFlags for a couple of hypervisors.
This wires up trivial versions (since neither hypervisor supports
managed save yet, they do not need to support any flags).

* src/vbox/vbox_tmpl.c (vboxDomainCreateXML): Update caller.
(vboxDomainUndefine): Move guts...
(vboxDomainUndefineFlags): ...to new function.
* src/xenapi/xenapi_driver.c (xenapiDomainUndefine)
(xenapiDomainUndefineFlags): Likewise.