Fix qemu-io option in featuretests

qemu-io refuses to open curl block device if no read-only option
given in qemu1.6, which will cause the feature tests always fail
even it supports. For details, please refer to:
https://github.com/qemu/qemu/commit/a7cea2

Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>