virttest.qemu_devices: Fix QStringDevice.cmdline problem

when False/None was set as cmdline this device always failed.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virttest.qemu_devices: Fix in-code OrderedDict

Fix the correct implementation of itervalues() and add iteritems().

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #647 from ldoktor/add_device2

virttest.qemu_devices: Additional two fixes