vfio/type1: Add vfio_group_iommu_domain()

Add the API for getting the domain from a vfio group. This could be used
by the physical device drivers which rely on the vfio/mdev framework for
mediated device user level access. The typical use case like below:

	unsigned int pasid;
	struct vfio_group *vfio_group;
	struct iommu_domain *iommu_domain;
	struct device *dev = mdev_dev(mdev);
	struct device *iommu_device = mdev_get_iommu_device(dev);

	if (!iommu_device ||
	    !iommu_dev_feature_enabled(iommu_device, IOMMU_DEV_FEAT_AUX))
		return -EINVAL;

	vfio_group = vfio_group_get_external_user_from_dev(dev);
	if (IS_ERR_OR_NULL(vfio_group))
		return -EFAULT;

	iommu_domain = vfio_group_iommu_domain(vfio_group);
	if (IS_ERR_OR_NULL(iommu_domain)) {
		vfio_group_put_external_user(vfio_group);
		return -EFAULT;
	}

	pasid = iommu_aux_get_pasid(iommu_domain, iommu_device);
	if (pasid < 0) {
		vfio_group_put_external_user(vfio_group);
		return -EFAULT;
	}

	/* Program device context with pasid value. */
	...

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>