iommufd/selftest: Require vdev_id when attaching to a nested domain

When attaching a device to a vIOMMU-based nested domain, vdev_id must be
present. Add a piece of code hard-requesting it, preparing for a vEVENTQ
support in the following patch. Then, update the TEST_F.

A HWPT-based nested domain will return a NULL new_viommu, thus no such a
vDEVICE requirement.

Link: https://patch.msgid.link/r/4051ca8a819e51cb30de6b4fe9e4d94d956afe3d.1741719725.git.nicolinc@nvidia.com


Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>