main-loop: Suppress I/O thread warning under qtest

We do not want to display the "I/O thread spun" warning for test cases
that run under qtest. The first attempt for this (commit
01c22f2cdd4fcf02276ea10f48253850a5fd7259) tested whether qtest_enabled()
was true.

Commit 21a24302e85024dd7b2a151158adbc1f5dc5c4dd correctly recognized
that just testing qtest_enabled() is not sufficient since there are some
tests that do not use the qtest accelerator but just the qtest character
device, and thus replaced qtest_enabled() by qtest_driver().

However, there are also some tests that only use the qtest accelerator
and not the qtest chardev; perhaps most notably the bash iotests.
Therefore, we have to check both qtest_enabled() and qtest_driver().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20161017180939.27912-1-mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
