pckbd: handle A20 IRQ as GPIO

The i8042 device has outgouing IRQ line A20. Currently the IRQ is referenced
by a pointer which normally is set during machine initialization. The pointer
is never changed at runtime. So common GPIO model can be applied to A20 IRQ
line. Note that checking for IRQ to be connected as in previous version
of code is not required because qemu_set_irq will do it.

Signed-off-by: Efimov Vasily <real@ispras.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
