loader: Fix leak on library objects

Currently, library objects are being leaked by run_main() on success of
osv::run() which, in addition to leaking memory, makes the dcache leak
directory entries that causes further problems.

Releasing library objects is fine as even dependent objects will be
released automatically. I have tested it, and dcache hasn't any leaked
dentries anymore.

This problem was found in our attempt to implement dentry hierarchy with
help for Avi Kivity.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Reviewed-by: Asias He <asias@cloudius-systems.com>
[ penberg: improve changelog ]
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>