util: rename qemu_open() to qemu_open_old()

util: rename qemu_open() to qemu_open_old()

We want to introduce a new version of qemu_open() that uses an Error
object for reporting problems and make this it the preferred interface.
Rename the existing method to release the namespace for the new impl.

Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Daniel P. Berrangé <[email protected]>