Remove the FV header assumption in variable driver.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dong Guo <guo.dong@intel.com>
Reviewed-by: Gao Liming <liming.gao@intel.com>
Reviewed-by: Zeng Star <star.zeng@intel.com>


git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@16564 6f19259b-4bc3-4df7-8a09-765794883524