KVM: MIPS: Use lddir/ldpte instructions to lookup gpa_mm.pgd

Loongson-3 can use lddir/ldpte instuctions to accelerate page table
walking, so use them to lookup gpa_mm.pgd.

Reviewed-by: Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Co-developed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <1590220602-3547-6-git-send-email-chenhc@lemote.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>