plugins: std -- Simplify __std_vprint_long_hex

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
plugins: std -- Add std_strtoul helper

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
test: Add std_strtoul usage

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>