This patch adds more comments to describe the structures and functions that are
public to the other components in the hypervisor. The comments are in
doxygen-style for document generation.
v2 -> v3:
* Reformat the flow in the doc for vhm_io_request.
v1 -> v2:
* Fix typos and inconsistencies in the comments.
* Wrap the text-based diagram in the doc for vhm_request in @verbatim
Tracked-On: #1595
Signed-off-by: Junjie Mao <junjie.mao@intel.com>