This creates a new AS object without any FlatView as
memory_region_transaction_commit() may want to reuse the empty FV.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20170921085110.25598-14-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>