Abstract some of the channel operations to allow for 'extra'
channels that do not have RX or TX queues.

- Try to assign a channel to each extra channel type that is enabled
  for the NIC, but gracefully degrade if we can't allocate sufficient
  MSI-X vectors
- Allow each extra channel type to generate its own channel name
- Allow channel types to disable reallocation and reinitialisation
  of their channels
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>