plugins: rework set_context() vmethod definition

plugins: rework set_context() vmethod definition

In bug 757598 was added the set_context() vmethod chain up in
GstVaapiPluginBase. But it is buggy, since the parent_class address is
assigned to the last element which called gst_vaapi_plugin_base_class_init().

No error has shown up since none of the element's base classes redefined
set_context() vmethod from GstElement, so always the correct function was
called. Still this code is wrong and this patch make it right.

Since set_context() is the same code, a macro is used to implement that code
in all the gst-vaapi elements.

https://bugzilla.gnome.org/show_bug.cgi?id=765368