For auto-detected devices, it is reasonable to assume that their state
changes from time to time, e.g. because a USB cable is unplugged.
Currently, whoever is responsible for syncing actual device state
to the one in the DeviceManager has to do the following:
    1. Get the IDevice from the DeviceManager.
    2. Clone it.
    3. Change the state in the copy.
    4. Re-add it to the DeviceManager, where the current object is
       being replaced (and another copy is made).
This is rather clumsy and unintuitive. Therefore, we now provide
DeviceManager::setDeviceState(), which turns the above into a one-liner
for client code and does not involve any copying.

Change-Id: Ic9f27e5c9f33c2b3a2f6de1976f12df1e78baf2e
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>