discover: Add an asynchronous version of load_url

A new load_url_async() function handles retrieving the resources by
forking an external process which currently blocks the parent waiting
for the process to complete. This patch modifies the load_*() functions
to handle it asynchronously by providing the exit callback and returning
without waiting for the child process to exit.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Neelesh Gupta <neelegup@linux.vnet.ibm.com>