The <inbound/> element to <bandwidth/> has several attributes from
which two are mandatory. Well, from two at least one has to be
present: @average or @floor or both. Instead of inventing crazy RNG
schema, let's make all the attributes optional there and rely on our
parsing code to correctly handle the situation.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>