backends/iommufd: Introduce iommufd_backend_alloc_veventq

Add a new helper for IOMMU_VEVENTQ_ALLOC ioctl to allocate a virtual event
queue (vEVENTQ) for a vIOMMU object.

Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
Message-id: 20260226084456.112142-2-skolothumtho@nvidia.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>