GDaemonFile: fix relative path handling to account for mount_prefix

If two files have two different origins (say, one from g_mount_get_root()
and one from g_file_new_for_uri()), the mount_spec they use could
expose a different mount_prefix, even if the represent the same URI
and network object.

This in particular fixes the handling of shadow mounts for dav
(which rewrites the mount_spec during mount to find the right prefix)

https://bugzilla.gnome.org/show_bug.cgi?id=696279