qemu: cpu hotplug: Fix error handling logic

qemu: cpu hotplug: Fix error handling logic

The cpu hotplug helper functions used negative error handling in a part
of them, although some code that was added later didn't properly set the
error codes in some cases. This would cause improper error messages in
cases where we couldn't modify the numa cpu mask and a few other cases.

Fix the logic by converting it to the regularly used pattern.