Use Dom0's resolv.conf unless nameserver is set.

If nameserver config option is not set (default value), copy Dom0's
/etc/resolv.conf on guest.
Provide a role to show how to customize /etc/resolv.conf in guest.

hooks/debian/40-setup-networking

@@ -143,6 +143,8 @@ E_O_STATIC
         for ns in $nameserver; do
             echo "nameserver $ns" >>${prefix}/etc/resolv.conf.old
         done
+    else
+        cp /etc/resolv.conf ${prefix}/etc/resolv.conf.old
     fi
 }