xenstore: add explicit memory context parameter to read_node()

xenstore: add explicit memory context parameter to read_node()

Add a parameter to xenstored read_node() function to explicitly
specify the memory context to be used for allocations. This will make
it easier to avoid memory leaks by using a context which is freed
soon.

When calling read_node() select a sensible memory context for the new
parameter by preferring a temporary one.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>