iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC

Introduce a new IOMMUFD_OBJ_VEVENTQ object for vIOMMU Event Queue that
provides user space (VMM) another FD to read the vIOMMU Events.

Allow a vIOMMU object to allocate vEVENTQs, with a condition that each
vIOMMU can only have one single vEVENTQ per type.

Add iommufd_veventq_alloc() with iommufd_veventq_ops for the new ioctl.

Link: https://patch.msgid.link/r/21acf0751dd5c93846935ee06f93b9c65eff5e04.1741719725.git.nicolinc@nvidia.com


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