discover/boot: Copy local paths before running boot hooks

We're seeing a crash when boot hooks are specifying new resources, as
boot_hook_update_param will write to a NULL struct load_url_result.

Instead of writing the updated values to the struct, copy the local
parts of the result to a separate string, which the boot hooks are free
to update.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>