tools/xenstore: introduce trace classes

tools/xenstore: introduce trace classes

Make the xenstored internal trace configurable by adding classes
which can be switched on and off independently from each other.

Define the following classes:

- obj: Creation and deletion of interesting "objects" (watch,
  transaction, connection)
- io: incoming requests and outgoing responses
- wrl: write limiting

Per default "obj" and "io" are switched on.

Entries written via trace() will always be printed (if tracing is on
at all).

Add the capability to control the trace settings via the "log"
command and via a new "--log-control" command line option.

Add a missing trace_create() call for creating a transaction.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>