malta: Fix regression (i8259 interrupts did not work)

Commit 5632ae46d5bda798e971dae48ebb318ac2c3686a passes the address
of i8259 to qemu_irq_proxy. i8259 is an auto variable with undefined
value outside of mips_malta_init.

This made the interrupt proxy unusable: either QEMU crashes, or
the interrupt handler was not called.

Ethernet for example no longer worked with MIPS Malta.

v2:
While v1 used a static variable for i8259, this patch introduces
a qdev for the malta machine. i8259 is now part of the device status.
This is a minimal qdev implementation to keep the patch small.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
