x86_iommu: check if machine has PCI bus

Starting qemu with
qemu-system-x86_64 -S -M isapc -device {amd|intel}-iommu
leads to a segfault. The code assume PCI bus is present and
tries to access the bus structure without checking.

Since Intel VT-d and AMDVI should only work with PCI, add a
check for PCI bus and return error if not present.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Mohammed Gamal <mgamal@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
