vfio: abort if an emulated iommu is used

vfio doesn't support guest iommus yet, indicate it to the user
by gently depositing a core on their disk.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Avi Kivity <avi.kivity@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>