pvcalls-back: do not return error on inet_accept EAGAIN

When the client sends a regular blocking accept request, the backend is
expected to return only when the accept is completed, simulating a
blocking behavior, or return an error.

Specifically, on EAGAIN from inet_accept, the backend shouldn't return
"EAGAIN" to the client. Instead, it should simply continue the wait.
Otherwise, the client will send another accept request, which will cause
another EAGAIN to be sent back, which is a waste of resources and not
conforming to the expected behavior. Change the behavior by turning the
"goto error" into a return.

Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>