xl/libxl: add gatewaydev/netdev to vif specification

xl/libxl: add gatewaydev/netdev to vif specification

This option is used by the vif-route hotplug script. A new more
descriptive name is used, "gatewaydev", but "netdev" is also supported
as a deprecated backwards compatible option.

This option was supported in the past, according to
http://wiki.xen.org/wiki/Vif-route, so we should also support it in
libxl.

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>