xcp-networkd: update to new command-line parsing convention

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Populate xcp-networkd config file with XenServer defaults

This should help us build new binaries which can adopt the dom0
defaults at runtime.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #87 from djs55/xcp-networkd-cmdliner

Adapt to new command-line argument parsing convention