Both xm and xl config have long supported specifying vif rate
limiting, e.g.

vif = [ 'mac=00:16:3E:74:3d:76,bridge=br0,rate=10MB/s' ]

Add support for mapping rate to and from <bandwidth> in the xenconfig
parser and formatter. rate is mapped to the required 'average' attribute
of the <outbound> element, e.g.

  <interface type='bridge'>
    ...
    <bandwidth>
      <outbound average='10240'/>
    </bandwidth>
  </interface>

Also add a unit test to check the conversion logic.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>