OvmfPkg/PlatformPei: Update ReserveEmuVariableNvStore

OvmfPkg/PlatformPei: Update ReserveEmuVariableNvStore

ReserveEmuVariableNvStore is updated with below 2 functions defined in
PlatformInitLib:
 - PlatformReserveEmuVariableNvStore
 - PlatformInitEmuVariableNvStore

PlatformInitEmuVariableNvStore works when secure boot feature is enabled.
This is because secure boot needs the EFI variables (PK/KEK/DB/DBX, etc)
and EmuVariableNvStore is cleared when OVMF is launched with -bios
parameter.

Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>