ioapic: Add support for qemu-kvm's vmstate v2

qemu-kvm carries the IOAPIC base address in its v2 vmstate. We only
support the default base address so far, and saving even that in the
device state was rejected.

Add a padding field to be able to read qemu-kvm's old state, but
increase our version to 3, indicating that we are not saving a valid
address. This also gives downstream the chance to change to stop
evaluating the base_address and move to v3 as well.

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