qemu-iotests: Filter out 'qemu-io> ' prompt

This removes "qemu-io> " prompt from qemu-io output in _filter_qemu_io,
and updates all the output files with the following command:

 cd tests/qemu-iotests && sed -i "s/qemu-io> //g" *.out

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>