configs/README: Fix and update QEMU config and demo description
The config file was no longer in sync with the setup described in the
README. Morever, we should include the HDA audio device in the QEMU
command line so that the pci-demo can be run. Update and enhance the
step-wise introduction in the README accordingly.
Include the virtio-9p-pci device in the config although we are not
referring to it in the README. It's an optional device at 00:1f.7, used
for local testings only.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>