Hello Tim,
great that you answered you question by your own and thanks for posting the answer!
I have just two minor remarks:
# make sure you have toolchain in your path export PATH=$PATH:/usr/local/genode-gcc/bin/
This is not needed. The build system knows the default location of the tool chain. I.e., it is defined in 'repos/base/etc/tools.conf'
# add libports, dde_linux, ports and qt4 repos vi etc/build.conf
Qt4 is not needed to execute the VirtualBox scenario. In fact, we deprecated Qt4 a while age when our port of Qt5 reached a stable state. Hence, 'repos/qt4' will eventually be removed from the source tree soon.
Cheers Norman