Initial VirtualBox support

This patch adds initial support for launching OSv instances under
VirtualBox.  The startup sequence is based on Asias He's vbox-demo.sh
script:

  https://github.com/asias/vbox-demo

We also configure VirtualBox serial console to use a Unix domain socket
and make Capstan connect to it and use VBoxHeadless to _not_ start up
the VM in background to work like QEMU does.

In future, we probably want "capstan up" like functionality Vagrant has
for launching VMs in the background.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>