bandwidth: Define schema and create documentation

bandwidth: Define schema and create documentation

Define new 'bandwidth' element with possible child element 'inbound'
and 'outbound' addressing incoming and outgoing traffic respectively:

<bandwidth>
  <inbound average='1000' peak='2000' burst='5120'/>
  <outbound average='500'/>
</bandwidth>

Leaving any element out means not to shape traffic in that
direction.
The units for average and peak (rate) are in kilobytes per second,
for burst (size) are just in kilobytes.
This element can be inserted into domain's 'interface' and
'network'.