CA-114673: Default create-time for a domain is 'now' not 0

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>

Description of the problem from John Else:

The events within xenopsd happen in approximately this order
(n.b. the sender and receiver are different threads in the
same xenopsd, as this is a localhost migration):

 1   Sender makes a request to receiver's receive_memory handler
 2   Receiver creates a new domain into which it will receive
     the memory (including writing the new domain's tree under
     /vm/<uuid>/<domid>)
 3   Sender initiates a save pointing at the fd used by the
     request from step 1
 4   Receiver initiates a restore using the other end of the
     above fd

Problem is, step 3 needs to know which of the two domains to
save, and there's no guarantee that step 3 starts after step 2
is finished. If the xenstore tree hasn't been written yet,
xenopsd defaults to reading the new domain's creation time as
zero, so will always choose it as the old domain to be saved.

Author: Jon Ludlam

xc/xenops_server_xen.ml

@@ -177,8 +177,9 @@ let di_of_uuid ~xc ~xs domain_selection uuid =
 			  try 
 			    xs.Xs.read (Printf.sprintf "/vm/%s/domains/%d/create-time" uuid' x.domid) |> Int64.of_string 
 			  with e ->
-			    warn "Caught exception trying to find creation time of domid %d (uuid %s)" x.domid uuid';
-			    0L
+                            warn "Caught exception trying to find creation time of domid %d (uuid %s)" x.domid uuid';
+                            warn "Defaulting to 'now'";
+                            Oclock.gettime Oclock.monotonic
 			in
 			compare (create_time a) (create_time b)
 		) possible in