OvmfPkg/LoongArchVirt: enable qemu uefi variable store support

Add QEMU_PV_VARS build option, when enables the firmware build will
support (and require) the qemu uefi variable store.

This also enables proper (as-in: actually being secure) secure boot
support.

Suggested-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Dongyan Qian <qiandongyan@loongson.cn>
Cc: Chao Li <lichao@loongson.cn>
Cc: Bibo Mao <maobibo@loongson.cn>
Cc: Xianglai Li <lixianglai@loongson.cn>
Cc: Bo Zhu <zhubo@loongson.cn>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Mike Beaton <mjsbeaton@gmail.com>