xl: allow specifying a default gatewaydev in xl.conf

xl: allow specifying a default gatewaydev in xl.conf

This adds a new global option in the xl configuration file called
"vif.default.gatewaydev", that is used to specify the default
gatewaydev to use when none is passed in the vif specification.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Tested-by: Ulf Kreutzberg <ulf.kreutzberg@hosteurope.de>
Cc: Ulf Kreutzberg <ulf.kreutzberg@hosteurope.de>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>