tools/xl: hide xenstore-features behind option

In order to be able to use "xl info" before Xenstore has been started
or after it has crashed, hide obtaining the available Xenstore features
behind the new option "-x".

Fixes: ecce7970cf ("tools/xl: add available Xenstore features to xl info output")
Reported-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>

docs/man/xl.1.pod.in

@@ -996,6 +996,10 @@ B<OPTIONS>
 
 List host NUMA topology information
 
+=item B<-x>, B<--xenstore>
+
+Show Xenstore features available.
+
 =back
 
 =item B<top>

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

@@ -743,7 +743,7 @@ condition.
 =back
 
 The features supported by the running Xenstore instance can be retrieved
-via the B<xl info> command in dom0.
+via the B<xl info -x> command in dom0.
 
 The default value is B<0xffffffff>, meaning that all possible Xenstore
 features are visible by the guest.