tests: virtio-9p: wait for completion in the test code

tests: virtio-9p: wait for completion in the test code

In order to test request cancellation, we will need to send multiple
requests and wait for the associated replies. Since we poll the ISR
to know if a request completed, we may have several replies to parse
when we detect ISR was set to 1.

This patch moves the waiting out of the reply parsing path, up into
the functional tests.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>