xl: use nic global default values in network-attach

xl: use nic global default values in network-attach

Introduce a new static function that will be used to set the initial nic
config based on the global defaults. This fixes a bug caused by
network-attach not using the default values set in xl.conf(5).

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>