BaseTools: GenFds get the Size info for FV image in the FD region

When the FV size is specify in the FD region, Tool generate the FV file
may not use the correct size.

Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=387


Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>