Returns the VM instance for the given block number.

def get_vm(self, at_header: BlockHeader=None) -> 'BaseVM':
        """
        Returns the VM instance for the given block number.
        """
        header = self.ensure_header(at_header)
        vm_class = self.get_vm_class_for_block_number(header.block_number)
        return vm_class(header=header, chaindb=self.chaindb)