MdeModulePkg Variable: Make sure no more than one Variable HOB

VariableHob may be built in PcdPeim (by PcdNvStoreDefaultValueBuffer)
or some platform module (by some tool).
The two solutions should not be co-exist.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>