python: Extract registered watch search logic from xspy_read_watch()

python: Extract registered watch search logic from xspy_read_watch()

When a watch fires, xspy_read_watch() checks whether the client has
registered interest in the path which changed and, if so, returns the
path and a client-supplied token.   The binding for xs_check_watch()
needs to do the same, so this patch extracts the search code into a
separate function.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>