qga: fix false negative argument passing

Function send_response(s, &qdict->base) returns a negative number
when any failures occured. But strerror()'s parameter cannot be
negative. Let's change the testing condition and pass '-ret' to
strerr().

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
