KVM: RISC-V: Unmap stage2 mapping when deleting/moving a memslot

Unmap stage2 page tables when a memslot is being deleted or moved.  It's
the architectures' responsibility to ensure existing mappings are removed
when kvm_arch_flush_shadow_memslot() returns.

Fixes: 9d05c1fe

 ("RISC-V: KVM: Implement stage2 page table programming")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>