backends/iommufd: Introduce iommufd_backend_alloc_viommu

Add a helper to allocate a viommu object.

Also introduce a struct IOMMUFDViommu that can be used later by vendor
IOMMU implementations.

Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Message-id: 20260126104342.253965-2-skolothumtho@nvidia.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>