OvmfPkg/RiscVVirt: Fix SCT memory allocation test case failure

OvmfPkg/RiscVVirt: Fix SCT memory allocation test case failure

Fix the UEFI memory range calculation by including the correct
stack memory range. Without this fix, SCT hangs in MemoryAllocation
test cases which call AllocateAddress().

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Andrei Warkentin <andrei.warkentin@intel.com>
Reported-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Andrei Warkentin <andrei.warkentin@intel.com>