The option sets cache mode used in the tests. "-nocache" is changed to
an alias to "-c none", and internally passes "-t none" to qemu-io.

Python scripts will make use of option this in the next commit.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NWenchao Xia <xiawenc@linux.vnet.ibm.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>