[Vala] code writer ignores the instance_pos attribute

[Victor Manuel Jáquez Leal <ceyusa gmail com>]

Yesterday I filled this bug (with its respective patch):

https://bugzilla.gnome.org/show_bug.cgi?id=609591

The problem is when setting the attribute instance_pos in a custom
vala for a vapi generation, the code writer doesn't honor the
attribute.

If everybody is ok with it, may I commit it?

vmjl