q35: ioapic: add support for emulated IOAPIC IR

q35: ioapic: add support for emulated IOAPIC IR

This patch translates all IOAPIC interrupts into MSI ones. One pseudo
ioapic address space is added to transfer the MSI message. By default,
it will be system memory address space. When IR is enabled, it will be
IOMMU address space.

Currently, only emulated IOAPIC is supported.

Idea suggested by Jan Kiszka and Rita Sinha in the following patch:

https://lists.gnu.org/archive/html/qemu-devel/2016-03/msg01933.html

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>