hw/loongarch/virt: Add field ram_end in LoongArchVirtMachineState

DRAM region is dynamically set and the last valid physical address region
with LoongArch Virt Machine. To record the last valid physical address,
field ram_end is added in structure LoongArchVirtMachineState. In future
end address of DRAM cannot exceed base addres of PCIE 64-bit MMIO region.

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