xen: reduce callback special cases

xen: reduce callback special cases

The callback struct is great when iterating through several
possibilities, but when calling a known callback, it's just
overhead.  We can make the direct call in those cases.

* src/xen/xen_driver.c (xenUnifiedOpen, xenUnifiedDomainSuspend)
(xenUnifiedDomainResume, xenUnifiedDomainDestroyFlags): Make
direct calls instead of going through callback.