netlink: simplify string conversion methods

There are multiple string handling needs shared between the netlink
modules. This patch unifies them to simplify all the modules.

Change-Id: I3cdd56cd243095d75a37c66d79e13c2715642d97
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27473
Reviewed-by: Dan Kenigsberg <danken@redhat.com>