BugLink: http://bugs.launchpad.net/bugs/1670508

Now SendReceive2 frees the first iov and returns a response buffer
in it that increases a code complexity. Simplify this by making
a caller responsible for freeing request buffer itself and returning
a response buffer in a separate iov.
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
(cherry picked from commit da502f7d)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>