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