We store AddressSpaceDispatch* in FlatView anyway so there is no need
to carry it from mem_add() to register_subpage/register_multipage.

This should cause no behavioural change.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20170921085110.25598-8-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>