irq: Slim conversion of qemu_irq to QOM

As a prequel to any big Pin refactoring plans, do an in-place conversion
of qemu_irq to an Object, so that we can reference it in link<> properties.

Signed-off-by: Andreas Färber <afaerber@suse.de>
[ PC Changes:
 * Removed array-alloctor ref counting logic (limit changes just to
 * single IRQ allocator)
 * Removed WIP marking from subject line
]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
