Return a single Node() instance by (form,valu) tuple.

async def getNodeByNdef(self, ndef):
        '''
        Return a single Node() instance by (form,valu) tuple.
        '''
        name, valu = ndef

        form = self.model.forms.get(name)
        if form is None:
            raise s_exc.NoSuchForm(name=name)

        norm, info = form.type.norm(valu)

        buid = s_common.buid((form.name, norm))

        async with await self.snap() as snap:
            return await snap.getNodeByBuid(buid)