virNetDevBandwidthParseRate: Reject negative values

virNetDevBandwidthParseRate: Reject negative values

https://bugzilla.redhat.com/show_bug.cgi?id=1022292

The following XML really does not make any sense:

<inbound average="-1" burst="-2" peak="-3" floor="-4"/>

There can't be a negative packet rate. Well, so far we haven't
assigned any meaning to it. So reject it unless users harm themselves,
because otherwise we turn the negative numbers into really big values.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
(cherry picked from commit 2a5d3f227df7be78449792103e0101a4b859c49b)