KVM: x86: Free arrays for old memslot when moving memslot's base gfn

Explicitly free the metadata arrays (stored in slot->arch) in the old
memslot structure when moving the memslot's base gfn is committed.  This
eliminates x86's dependency on kvm_free_memslot() being called when a
memslot move is committed, and paves the way for removing the funky code
in kvm_free_memslot() that conditionally frees structures based on its
@dont param.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>