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.