Updates to blktap driver and user code.

Updates to blktap driver and user code.

Mostly this makes the tap code work again with all of the changes that
have happened to the block drivers recently.  We now use a shared page
per VBD (to the driver), and handle control information through the
store.  The taplib interfaces have changed to be based around per-vbd
data structures that you can attach arbitrary handlers for.

There is also initial code for a user-level blockback driver, which
aims to get around the use of loopbacks for file-based vbds.  Still
plenty of work to do here -- this is a working incremental checkin and
I'm away from this for the next four weeks.

Signed-off-by: Andrew Warfield <andrew.warfield@cl.cam.ac.uk>