target/loongarch: Correct maximum physical address in KVM mode

target/loongarch: Correct maximum physical address in KVM mode

On 3A5000 system, the physical address space width for host is 48,
however 47 bit for KVM VM. For KVM VM, size of physical address space is
the same with that of virtual user space address.

Here modify physical address space width with 47 bit in KVM mode.

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