Using C99 initializers and xen-specific prefixes will make it
so that future patches are less likely to add callback members
to the xenUnifiedDriver struct, since the goal is to get rid
of the callback struct in the first place.

* src/xen/xen_driver.h (xenUnifiedDriver): Rename all struct
members, to make it obvious which ones are still in use.
* src/xen/xen_driver.c: Update all callers.
* src/xen/xen_hypervisor.c (xenHypervisorDriver): Rewrite with C99
initializers.
* src/xen/xend_internal.c (xenDaemonDriver): Likewise.
* src/xen/xs_internal.c (xenStoreDriver): Likewise.
* src/xen/xm_internal.c (xenXMDriver): Likewise.
* src/xen/xen_inotify.c (xenInotifyDriver): Likewise.