MdeModulePkg DxeCore: Handle multiple FV images in one FV file

MdeModulePkg DxeCore: Handle multiple FV images in one FV file

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1131

PI spec and BaseTools support to generate multiple FV images
in one FV file.
This patch is to update DxeCore to handle the case.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Jiewen Yao <jiewen.yao@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>