support for TAP devices

For some testing scenarios, e.g. multi-machine testing, advanced networking configuration is needed. Following patch add support to start QEMU with TAP device networking instead of default user networking. 

Patch also changes QEMU parameters from -net to -netdev as according to QEMU doc, -net is considered obsolete.

Note that TAP devices require some additional configuration steps on worker hosts before they can be used, documented in doc/networking.md.

There is one additional small patch for openQA worker which is responsible to set proper TAPDEV, I will create pull request there once this pull request is resolved.