oxenstored: fix ABI breakage introduced in Xen 4.9.0

dbc84d2983 (Xen >= 4.9.0) deleted XS_RESTRICT
from oxenstored, which caused all the following opcodes to be shifted by 1:
reset_watches became off-by-one compared to the C version of xenstored.

Looking at the C code the opcode for reset watches needs:
XS_RESET_WATCHES = XS_SET_TARGET + 2

So add the placeholder `Invalid` in the OCaml<->C mapping list.
(Note that the code here doesn't simply convert the OCaml constructor to
 an integer, so we don't need to introduce a dummy constructor).

Igor says that with a suitably patched xenopsd to enable watch reset,
we now see `reset watches` during kdump of a guest in xenstored-access.log.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Tested-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
(cherry picked from commit a6ed77f1e0)
(cherry picked from commit 9503a6df19)

tools/ocaml/libs/xb/op.ml

@@ -28,7 +28,7 @@ let operation_c_mapping =
            Transaction_end; Introduce; Release;
            Getdomainpath; Write; Mkdir; Rm;
            Setperms; Watchevent; Error; Isintroduced;
-           Resume; Set_target; Reset_watches |]
+           Resume; Set_target; Invalid; Reset_watches |]
 let size = Array.length operation_c_mapping
 
 let array_search el a =