xenstore: add small default data buffer to internal struct

xenstore: add small default data buffer to internal struct

Instead of always allocating a data buffer for incoming or outgoing
xenstore wire data add a small buffer to the buffered_data structure
of xenstored. This has the advantage that especially sending simple
response messages like errors or "OK" will no longer need allocating
a data buffer. This requires adding a memory context where the
allocated buffer was used for that purpose.

In order to avoid allocating a new buffered_data structure for each
response reuse the structure of the original request. This in turn
will avoid any new memory allocations for sending e.g. an ENOMEM
response making it possible to send it at all. To do this the
allocation of the buffered_data structure for the incoming request
must be done when a new request is recognized instead of doing it
when accepting a new connect.

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