iotests: Filter out "I/O thread spun..." warning

Filter out the "main loop: WARNING: I/O thread spun for..." warning from
qemu output (it hardly matters for code specifically testing I/O).

Furthermore, use _filter_qemu in all the custom functions which run
qemu.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/tests/qemu-iotests/099 b/tests/qemu-iotests/099
index 948afff..80f3d9a 100755
--- a/tests/qemu-iotests/099
+++ b/tests/qemu-iotests/099
@@ -57,7 +57,7 @@
     # Get the "file": "foo" entry ($foo may only contain escaped double quotes,
     # which is how we can extract it)
     do_run_qemu "$@" 2>&1 | _filter_testdir | _filter_imgfmt | _filter_qmp \
-        | grep "drv0" \
+        | _filter_qemu | grep "drv0" \
         | sed -e 's/^.*"file": "\(\(\\"\|[^"]\)*\)".*$/\1/' -e 's/\\"/"/g'
 }