harness: add support for pushing BufferList

Submitted by Miguel París Díaz @mparisdiaz

Link to original bug (#786800)

Description

Created attachment 358399
harness: add support for pushing BufferList

Providing support for pushing BufferList in Harness is quite useful to test elements being able to handle it.
In this way we could easily test that the behavior is the expected, that there is not memory leaks, etc.

Patch 358399, "harness: add support for pushing BufferList":
0001-harness-add-support-for-pushing-BufferList.patch