qom: Make object_resolve_path_component() path argument const

A usage with a hardcoded partial path such as

  object_resolve_path_component(obj, "foo")

is totally valid but currently leads to a compilation error. Fix this.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
