If the is_write argument is true, address_space_rw writes to memory
and thus reads from the buffer.  The opposite holds if is_write is
false.  Fix the model.

Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>