virtio-gpu: disallow vIOMMU

virtio-gpu has special code path that bypassed vIOMMU protection.  So
for now let's disable iommu_platform for the device until we fully
support that (if needed).

After the patch, both virtio-vga and virtio-gpu won't allow to boot with
iommu_platform parameter set.

CC: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-id: 20180131040401.3550-1-peterx@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>