Switch block device setup/teardown over to xenbus/xenstore.

Switch block device setup/teardown over to xenbus/xenstore.
Remove device probing support over the device channel.
g/c all the control message code.
Simplify and cleanup internal function calling code (explicit arguments
instead of passing structs around).
Also includes:
Changed "readonly" to "read-only". "backend" is still not hyphenated, but
that seems to be fairly common usage.
Block backends use xenbus_dev_error, making debugging nicer.
The block front end is told explicitly what domid to use to talk to the
backend. There's speculation that fake domid-like tokens may be given
for cluster purposes in future, so deriving from the backend dir name is
not guaranteed to work. Backends still derive front-end ids, using a
xenbus helper (xenbus_uuid_to_domid).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>