coverity: fix address_space_rw model 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>