SUPPORT.md: add xen-9pfsd

Add a support statement for the new xen-9pfsd backend. Set it to
"Experimental", as some functionality for Linux guests is missing
(only tested to work with Xenstore-stubdom).

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

SUPPORT.md

@@ -665,6 +665,7 @@ there is currently no xl support.
 ### PV 9pfs (backend)
 
     Status, QEMU: Tech Preview
+    Status, xen-9pfsd: Experimental
 
 ### PVCalls (backend)