prior to this patch, the configured vpair interfaces would not
be saved to the config file on 'write file'.
Change-Id: Iff6551318534a3717c374060082147f17b925a21
The idea is to generate a pair of virtual E1 interfaces (each with
identical number of lines), where each line A:n is connected to line B:n
of the pair and vice-versa.
This allows to test E1 using applications back to back against each
other, without any physical E1 circuits in between.
Change-Id: If42c959556b17d543762546eb45dd69d25f715f2