Documentation: virtual: Add toctree hooks

Added toctree hooks for indexing. Hooks added only for newly added
files.

The hook for the top of the tree will be added in a later patch series
when a few more substantial changes have been added.

Signed-off-by: Luke Nowakowski-Krijger <lnowakow@eng.ucsd.edu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>