dbus 1.2.24 (on RHEL 6) lacks DBUS_TYPE_UNIX_FD; but as we aren't
trying to pass one of those anyways, we can just drop support for
it in our wrapper.  Solves this build error introduced in commit
834c9c94:

  CC     libvirt_util_la-virdbus.lo
util/virdbus.c:242: error: 'DBUS_TYPE_UNIX_FD' undeclared here (not in a function)

* src/util/virdbus.c (virDBusBasicTypes): Drop support for unix fds.
Signed-off-by: NEric Blake <eblake@redhat.com>