add PyGtk demo and fixed systemd
qemu-demo-start: add Qt5 demo
qemu-demo-start: fix image path
qemu-demo-start: add Grsecurity demo for x86.
qemu-demo-start: add SMACK demo for x86.
qemu-demo-start: add configuration file support.

The demo configurations are not longer hard-coded in the script, but
read from a configuration file, which default location is
$HOME/.config/qemu-demo-start.conf.