oxenstored: avoid leading slash in paths in saved store state

oxenstored: avoid leading slash in paths in saved store state

Internally, paths are represented as lists of strings, where
  * path "/" is represented by []
  * path "/local/domain/0" is represented by ["local"; "domain"; "0"]
(see comment for Store.Path.t).

However, the traversal function generated paths like
    [""; "local"; "domain"; "0"]
because the name of the root node is "". Change it to generate paths
correctly.

Furthermore, the function passed to Store.dump_fct would render the node
"foo" under the path [] as "//foo". Change this to return "/foo".

Signed-off-by: Jonathan Davies <jonathan.davies@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Christian Lindig <christian.lindig@citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>