xenstore: let write_node() and some callers return errno

xenstore: let write_node() and some callers return errno

Instead of setting errno and returning true or false return the error
value directly.

In order to ensure all call sites have been changed according to the
modification rename the functions to xs_*.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>