hw/loongarch/virt: Replace RSDT with XSDT table

XSDT table is introduced in ACPI Specification 5.0, it supports 64-bit
address in the table. There is LoongArch system support from ACPI
Specification 6.4 and later, XSDT is supported by LoongArch system.

Here replace RSDT with XSDT table.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>