extract switch_to_qemu_if_required() function

This refactoring also fixes a bug.
extract batch_build_proposals()
heat demo: make VAGRANT_CONFIG_FILE externally tweakable
HA demo: make use of vagrant config file explicit
check VAGRANT_CONFIG_FILE is valid
make it easier to build HA demos with only 8GB of RAM
add some sanity checks for VirtualBox
do hypervisor checks earlier
document some gotchas with VirtualBox and Vagrant usage