iommu/vt-d: Change scope lists to struct device, bus, devfn

It's not only for PCI devices any more, and the scope information for an
ACPI device provides the bus and devfn so that has to be stored here too.

It is the device pointer itself which needs to be protected with RCU,
so the __rcu annotation follows it into the definition of struct
dmar_dev_scope, since we're no longer just passing arrays of device
pointers around.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>