MdeModulePkg DxeCore: Support USED_SIZE FV_EXT_TYPE

MdeModulePkg DxeCore: Support USED_SIZE FV_EXT_TYPE

The USED_SIZE FV_EXT_TYPE is introduced by PI 1.6 spec.

The EFI_FIRMWARE_VOLUME_EXT_ENTRY_USED_SIZE_TYPE can be used to find
out how many EFI_FVB2_ERASE_POLARITY bytes are at the end of the FV.

When the FV gets shadowed into memory you only need to copy the used
bytes into memory and fill the rest of the memory buffer with the
erase value.

Cc: Liming Gao <liming.gao@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>