qemu-iotests: do not buffer the test output

qemu-iotests: do not buffer the test output

Instead of buffering the test output into a StringIO, patch it on
the fly by wrapping sys.stdout's write method.  This can be
done unconditionally, even if using -d, which makes execute_unittest
a bit simpler.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Tested-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20210323181928.311862-2-pbonzini@redhat.com>
Message-Id: <20210503110110.476887-2-pbonzini@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>