We're about to support a third vIOMMU for x86, virtio-iommu which
doesn't inherit X86IOMMUState. Move the IOMMU singleton into
PCMachineState, so it can be shared between all three vIOMMUs.

The x86_iommu_get_default() helper is still needed by KVM and IOAPIC to
fetch the default IRQ-remapping IOMMU. Since virtio-iommu doesn't
support IRQ remapping, this interface doesn't need to change for the
moment. We could later replace X86IOMMUState with an "IRQ remapping
IOMMU" interface if necessary.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Message-Id: <20211026182024.2642038-4-jean-philippe@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>