Make NIC setup more flexible in case of Xen

For every NIC write down a file that contains NIC's
info (MAC, mode, link, network details), that can
be sourced by any vif script.  The file location is:
/var/run/ganeti/xen-hypervisor/nic/<domname>/<nicidx>.

This file is created upon cfg file creation and before
starting the instance and can be sourced by an external
script.

Cleanup NIC dir when removing config files. Upon Xen
configuration file removal, remove NIC dir as well.

Add new hv param vif_script that allows execution of
user defined networking script and overrides the one
xend is configured with in xend-config.sxp

Ensure that all run dirs exist for Xen hypervisor

Xen hypervisor uses runtime files to store each NIC's related info.
We need to check if parent dirs exist as well.

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