schema: consolidate RNG for all hostdev <driver> elements

schema: consolidate RNG for all hostdev <driver> elements

The exact same element can appear in <hostdev> and <interface
type='hostdev'>, and nearly identical in <network> and <networkport>
(these latter two don't include "xen" as a possible driver, but that's
coincidental - there's no reason Xen couldn't also use the VF pools in
virtual networks, it just doesn't).

This patch modifies all 4 to use the same <ref name="hostdevDriver"/>
so that it is simpler to add something new.

A side effect of this patch is that the grammar for the <interface>
element in domain XML has been tightened up a bit - previously it was
accepted by the schema (but nonsensical) to have virtio and network
interface options specified; as a part of making the two different
<driver> choices each a complete element (rather than each being a
collection of attributes and subelements) these extra
attributes/subelements that were irrelevant to the hostdev-type
<driver> were made to be valid only for an emulated interface's
<driver>.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>