RHEL 6.3 uses dbus-devel-1.2.24, which lacked support for the
DBUS_TYPE_UNIX_FD define (contrast with Fedora 18 using 1.6.8).
But since it is an older dbus, it also lacks support for shutdown
inhibitions as provided by newer systemd.

Compilation failure introduced in commit 31330926.

* src/rpc/virnetserver.c (virNetServerAddShutdownInhibition):
Compile out if dbus is too old.