bandwidth: Integrate bandwidth into portgroups

Every DomainNetDef has a bandwidth, as does every portgroup.
Whenever a DomainNetDef of type NETWORK is about to be used, a call is
made to networkAllocateActualDevice(). This function chooses the "best"
bandwidth object and places it in the DomainActualNetDef.
From that point on, whenever some code needs to use the bandwidth data
for the interface, it's retrieved with virDomainNetGetActualBandwidth(),
which will always return the "best" info as determined in the
previous step.

docs/formatnetwork.html.in

@@ -129,6 +129,8 @@
         numbers, The units for <code>average</code> and <code>peak</code> attributes
         are kilobytes per second, and for the <code>burst</code> just kilobytes.
         The rate is shared equally within domains connected to the network.
+        Moreover, <code>bandwidth</code> element can be included in
+        <code>portgroup</code> element.
         <span class="since">Since 0.9.4</span>
       </p>