tools/libs/store: add support to use watches with a depth parameter

Add a new xs_watch_depth() function to libxenstore allowing to limit
the scope of a Xenstore watch. It can be used only in case Xenstore is
supporting the XENSTORE_SERVER_FEATURE_WATCHDEPTH feature.

For convenience add a xs_watch_try_depth() wrapper, which will call
xs_watch_depth() if supported and xs_watch() otherwise.

Cache the supported features of Xenstore in order not having to get
them from Xenstore for each call of one of the new functions.

While touching xs.c fix a whitespace issue there.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>

docs/man/xl.cfg.5.pod.in

@@ -740,6 +740,12 @@ Xenstore is capable to reconnect to a guest.
 Xenstore will present an error value in case it disconnects due to an error
 condition.
 
+=item B<0x00000004>
+
+Xenstore supports to set watches with a limited depth (depth 0 matches
+only the watched node, depth 1 matches the node and its direct children,
+etc.).
+
 =back
 
 The features supported by the running Xenstore instance can be retrieved