iotests: Let 216 make use of qemu-io's exit code

iotests: Let 216 make use of qemu-io's exit code

As a showcase of how you can use qemu-io's exit code to determine
success or failure (same for qemu-img), this test is changed to use
qemu_io_silent() instead of qemu_io(), and to assert the exit code
instead of logging the filtered result.

One real advantage of this is that in case of an error, you get a
backtrace that helps you locate the issue in the test file quickly.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180509194302.21585-6-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>