qemu-iotests: Add missing -machine accel=qtest

qemu-iotests: Add missing -machine accel=qtest

A basic set of qemu options is initialised in ./common:

    export QEMU_OPTIONS="-nodefaults -machine accel=qtest"

However, two test cases (172 and 186) overwrite QEMU_OPTIONS and neglect
to manually set '-machine accel=qtest'. Add the missing option for 172.
186 probably only copied the code from 172, it doesn't actually need to
overwrite QEMU_OPTIONS, so remove that in 186.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>