OvmfPkg/PlatformPei: don't allocate reserved mem varstore if SMM_REQUIRE

OvmfPkg/PlatformPei: don't allocate reserved mem varstore if SMM_REQUIRE

For the emulated variable store, PlatformPei allocates reserved memory (as
early as possible, so that the address remains the same during reboot),
and PcdEmuVariableNvStoreReserved carries the address to
EmuVariableFvbRuntimeDxe.

However, EmuVariableFvbRuntimeDxe is excluded from the SMM_REQUIRE build,
and then noone consumes PcdEmuVariableNvStoreReserved. Don't waste
reserved memory whenever that's the case.

(Even a dynamic default for PcdEmuVariableNvStoreReserved would be
unnecessary; but that way the PcdSet64S() call in the
ReserveEmuVariableNvStore() function doesn't compile.)

Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>