intel-iommu: Support IOMMU groups

Add IOMMU group support to Intel VT-d code.  This driver sets up
devices ondemand, so make use of the add_device/remove_device
callbacks in IOMMU API to manage setting up the groups.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>