MinPlatformPkg: Fix SetLargeVariable fail issue.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4454



On Server platform, when the large variable "FspNvsBuffer" is already
in the UEFI variable store and the remaining variable storage space is
less than the large variable size, and also not in OS runtime, then we
need to add the size of the current data that will end up being
replaced by the new data to the remaining space before we decide that
there is not enough space to store the large variable.

Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Eric Dong <eric.dong@intel.com>
Co-authored-by: Xiaoqiang Zhang <xiaoqiang.zhang@intel.com>
Signed-off-by: Miki Shindo <miki.shindo@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>