libxl: rename vifs to nics

libxl: rename vifs to nics

This change renames functions and struct values that used to contain
vifs in their names to nics, that provides a more clear name to
define network interfaces without referring to the backend that is
behind them.

This is not a functional change.

Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>