Start using doxygen for OSv's API documention

This patch begins to document OSv's native (non-Linux-compatible) C++ APIs
using Doxygen. With doxygen, functions, classes, and other APIs are
documented using comments inside the source code, similarly to the way
Javadoc is used in the Java world.

After this patch, running "doxygen" generates the documentation in HTML
form (and also ugly ones in LaTeX...) in the "doxyout" directory.
After running "doxygen" in the OSv top-level directory, open your browser to
file://.../doxyout/html/index.html to view this documentation.

This patch adds a doxygen configuration file, "Doxyfile", and documents
three functions in the osv namespace: osv::halt(), osv::poweroff(),
osv::halt().

The format of the resulting documentation is not perfect, but it's not
as bad as I feared, and also the documentation doesn't uglify the code
too much (it mainly impacts the header files), so I now believe it will
better for us than writing man pages (an avenue which I explored
previously).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>