vl: Factor configure_blockdev() out of main()

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190308131445.17502-7-armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>