Re-add the oldstyle migration-via-fd-passing interface

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Use the same xs client for all watching

We don't need to use a different client any more as the callbacks
are issued on a different thread. Also, some of the watches were
set on a client without a callback, which caused some bugs.

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