intel_iommu: Add support for Extended Interrupt Mode

intel_iommu: Add support for Extended Interrupt Mode

As neither QEMU nor KVM support more than 255 CPUs so far, this is
simple: we only need to switch the destination ID translation in
vtd_remap_irq_get if EIME is set.

Once CFI support is there, it will have to take EIM into account as
well. So far, nothing to do for this.

This patch allows to use x2APIC in split irqchip mode of KVM.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
[use le32_to_cpu() to retrieve dest_id]
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>