hw/loongarch/virt: Set MADT revision to 6

The LoongArch virt machine emits Core PIC, EIO PIC, MSI PIC and BIO PIC
subtables, but advertises MADT revision 1. Revision 1 predates these
LoongArch interrupt controller structures.

ACPI 6.5 introduced the LoongArch interrupt controller structures and
defined MADT revision 6. ACPI 6.6 raises the MADT revision to 7 for the
additional RISC-V interrupt controller structures, while leaving the
LoongArch structures unchanged.

Since the virt machine emits only the LoongArch structures defined by
ACPI 6.5, set the MADT header revision to 6, the minimum revision that
describes the table contents.

Fixes: 735143f10d3 ("hw/loongarch: Add acpi ged support")
Signed-off-by: Dongyan Qian <qiandongyan@loongson.cn>
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Bibo Mao <maobibo@loongson.cn>