xen: make direct call when there is only one subdriver

xen: make direct call when there is only one subdriver

No need to use a for loop if we know there is exactly one client.
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 used exactly once.  The next patch will ensure that we
don't reintroduce uses of these callbacks.

* src/xen/xen_driver.c (xenUnifiedClose): Call close
unconditionally, to match xenUnifiedOpen.
(xenUnifiedNodeGetInfo, xenUnifiedDomainCreateXML)
(xenUnifiedDomainSave, xenUnifiedDomainRestore)
(xenUnifiedDomainCoreDump, xenUnifiedDomainUpdateDeviceFlags):
Make direct call to lone implementation.
* src/xen/xend_internal.h (xenDaemonDomainCoreDump)
(xenDaemonUpdateDeviceFlags, xenDaemonCreateXML): Add prototypes.
* src/xen/xend_internal.c (xenDaemonDomainCoreDump)
(xenDaemonUpdateDeviceFlags, xenDaemonCreateXML): Export.