Fix Xen's NIC file

Xen exports all environment variables to a file so that an
external script (see vif_script hvparam) can source it.
In case of multiple values per variable (e.g. TAGS), we
should use XXX="a b c" instead of XXX=a b c. Fix this.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Hrvoje Ribicic <riba@google.com>