xen: drop unused callbacks

xen: drop unused callbacks

Found by:

for f in $(sed -n 's/.*Drv[^ ]* \([^;]*\);.*/\1/p' src/xen/xen_driver.h)
do
  git grep "\(\.\|->\)$f\b" src/xen
done | cat

and looking through the resulting list to see which callback struct
members are still necessary.

* src/xen/xen_driver.h (xenUnifiedDriver): Drop all callbacks that
are only used directly.
* src/xen/xen_hypervisor.c (xenHypervisorDriver): Shrink list.
* src/xen/xen_inotify.c (xenInotifyDriver): Likewise.
* src/xen/xend_internal.c (xenDaemonDriver): Likewise.
* src/xen/xm_internal.c (xenXMDriver): Likewise.
* src/xen/xs_internal.c (xenStoreDriver): Likewise.