conf: re-situate <bandwidth> element in <interface>

conf: re-situate <bandwidth> element in <interface>

This moves the call to virNetDevBandwidthFormat() in
virDomainNetDefFormat() to be called right after the call to
virNetDevVPortProfileFormat(), so that a single chunk of that function
can be placed inside an if that conditionally calls
virDomainActualNetDefContentsFormat() instead (next patch). The
re-ordering necessitates modifying a couple of test data files.