Refactor NIC configuration scripts

Add new script vif-ganeti that could be used instead of
default vif-bridge in order NIC customization can be more
flexible.

Extract common code from kvm-ifup and insert it in a new
file net-common that is being sourced by kvm-ifup and
vif-ganeti and located under package lib dir (/usr/lib/ganeti/).

Install vif-ganeti properly with make.

Add a mkdir and ln rule in install-exec-hook so that
the /etc/xen/scripts dir is created and the vif-ganeti
scripts gets properly linked inside it.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Thomas Thrainer <thomasth@google.com>