iommufd/selftest: Add IOPF support for mock device

iommufd/selftest: Add IOPF support for mock device

Extend the selftest mock device to support generating and responding to
an IOPF. Also add an ioctl interface to userspace applications to trigger
the IOPF on the mock device. This would allow userspace applications to
test the IOMMUFD's handling of IOPFs without having to rely on any real
hardware.

Link: https://lore.kernel.org/r/20240702063444.105814-10-baolu.lu@linux.intel.com
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>