OvmfPkg/RiscVVirt: Fix issues in VarStore Blockmap config

The block size configuration of Blockmap does not match that in Qemu
VirtNorFlash, which causes variable data to be written into FtwWorkBlock
by mistake, resulting in data loss during reboot. Fix it and update
new checksum value.

Signed-off-by: Qingyu Shang <2931013282@sjtu.edu.cn>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>