i8259: don't abort when trying to use level sensitive irqs

i8259: don't abort when trying to use level sensitive irqs

This is a guest-triggerable error, as seen when using Xenix 2.3.4.
Replace hw_error by LOG_UNIMPL, so that guests can continue.
With this patch, I can install and use Xenix 2.3.4a without any problem.
I can also start installation of Xenix 2.3.4q, but it fails due to not
finding an hard disk.

Signed-off-by: Hervé Poussineau <[email protected]>
Acked-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>