Change the handling of port F0h writes and FPU exceptions to implement IGNNE.
The implementation mixes a bit what the chipset and processor do in real
hardware, but the effect is the same as what happens with actual FERR#
and IGNNE# pins: writing to port F0h asserts IGNNE# in addition to lowering
FP_IRQ; while clearing the SE bit in the FPU status word deasserts IGNNE#.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>