SUPPORT.md: clarify support of untrusted driver domains with oxenstored

Add a support statement for the scope of support regarding different
Xenstore variants. Especially oxenstored does not (yet) have security
support of untrusted driver domains, as those might drive oxenstored
out of memory by creating lots of watch events for the guests they are
servicing.

Add a statement regarding Live Update support of oxenstored.

This is part of XSA-326.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Christian Lindig <christian.lindig@citrix.com>
(cherry picked from commit c7bc20d8d1)

SUPPORT.md

@@ -175,6 +175,17 @@ Support for running qemu-xen device model in a linux stubdomain.
 
     Status: Tech Preview
 
+## Xenstore
+
+### C xenstored daemon
+
+    Status: Supported
+
+### OCaml xenstored daemon
+
+    Status: Supported
+    Status, untrusted driver domains: Supported, not security supported
+
 ## Toolstack/3rd party
 
 ### libvirt driver for xl