iommu: Check the result of iommu_group_get() for NULL

The result of iommu_group_get() was being blindly used in both
attach and detach which results in a dereference when trying
to work with an unknown device.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>