Add a helper function for getting the root PD addr and cleanup join the
two VM related functions and cleanup the function name.

No functional change.
Signed-off-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NHuang Rui <ray.huang@amd.com>
Reviewed-by: NJunwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>