Change watches: operations block until everyone has acked.

Change watches: operations block until everyone has acked.
Watch events are no longer sent to self
Watches no longer take a priority
async and asyncwait commands for xs_test, now we need to continue
despite blocking ops.
Print test name at end of verbose run on failure.
Use --trace-file arg to xenstored when testing
Signed-off-by: Rusty Russel <rusty@rustcorp.com.au>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>