virrandom: Accept "nodedev" driver in virRandomGenerateWWN()

virrandom: Accept "nodedev" driver in virRandomGenerateWWN()

The virRandomGenerateWWN() is used solely by nodedev driver to
autogenerate WWNN and WWNP when parsing a nodedev XML. Now, the
idea was (at least during monolithic daemon) that depending on
which hypervisor driver called the nodedev XML parsing (and
virRandomGenerateWWN() under the hood) the corresponding OUI is
used (e.g. "001a4a" for the QEMU driver).

But in era of split daemons things are not that easy. We do not
know which hypervisor driver called us. And there might be no
hypervisor driver at all - users are allowed to connect to
individual drivers directly (e.g. "nodedev:///system").

In this case, we can't use proper OUI. Well, do the next best
thing: pick one (QUMRANET_OUI).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>