x86_iommu: check if machine has PCI bus

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 <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Signed-off-by: Mohammed Gamal <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>