Give VirtualBox page weighting

The weighting of the pages is used to order them on the site, with pages of
equal weight being sorted by date. The default weight in Hugo is 0, the lowest
weight. The virtualbox page currently doesn't specify a weight, this results in
the VirtualBox page being dropped to the bottom of the sorting even though it's
one of the newest pages.

Providing pages with our own default weight gives us the flexibility to demote
content in the future that we want to keep, but wish to have lower priority.
For now, give the VirtualBox page the same weighting as the other pages so that
it doesn't sink to the bottom.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>