tools/xenstored: Fix off-by-one in dump_state_nodes()

tools/xenstored: Fix off-by-one in dump_state_nodes()

The maximum path length supported by Xenstored protocol is
XENSTORE_ABS_PATH_MAX (i.e 3072). This doesn't take into account the
NUL at the end of the path.

However, the code to dump the nodes will allocate a buffer
of XENSTORE_ABS_PATH. As a result it may not be possible to live-update
if there is a node name of XENSTORE_ABS_PATH.

Fix it by allocating a buffer of XENSTORE_ABS_PATH_MAX + 1 characters.

Take the opportunity to pass the max length of the buffer as a
parameter of dump_state_node_tree(). This will be clearer that the
check in the function is linked to the allocation in dump_state_nodes().

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>