main-loop: suppress warnings under qtest

commit 01c22f2c ("main-loop: Suppress
"I/O thread spun" warnings for qtest") doesn't actually disable the
warning for everyone since some tests don't run under the qtest
accelerator.

Check qtest_driver instead.

Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <1448882964-22433-1-git-send-email-mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>