i8254: Pass alternative IRQ output object on initialization

i8254: Pass alternative IRQ output object on initialization

HPET legacy emulation will require control over the PIT IRQ output. To
enable this, add support for an alternative IRQ output object to the PIT
factory function. If the isa_irq number is < 0, this object will be
used.

This also removes the IRQ number property from the PIT class as we now
use a generic GPIO output pin that is connected by the factory function.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>