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>
