iommufd/selftest: Add coverage for vdevice tombstone

iommufd/selftest: Add coverage for vdevice tombstone

This tests the flow to tombstone vdevice when idevice is to be unbound
before vdevice destruction. The expected results of the tombstone are:

 - The vdevice ID can't be reused anymore (not tested in this patch).
 - Even ioctl(IOMMU_DESTROY) can't free the vdevice ID.
 - iommufd_fops_release() can still free everything.

Link: https://patch.msgid.link/r/20250716070349.1807226-8-yilun.xu@linux.intel.com
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>