vfio/iommufd: Don't initialize nor set a HOST_IOMMU_DEVICE with mdev

vfio/iommufd: Don't initialize nor set a HOST_IOMMU_DEVICE with mdev

mdevs aren't "physical" devices and when asking for backing IOMMU info, it
fails the entire provisioning of the guest. Fix that by skipping
HostIOMMUDevice initialization in the presence of mdevs, and skip setting
an iommu device when it is known to be an mdev.

Cc: Zhenzhong Duan <zhenzhong.duan@intel.com>
Fixes: 930589520128 ("vfio/iommufd: Implement HostIOMMUDeviceClass::realize() handler")
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>