hhvm: fix status

Unluckily enough, HHVM admin interface speaks FastCGI but... it doesn't
understand it, so we should ask apache the exact same url we will then
proxy to the HHVM admin interface (meh!). So, converting this to a
virtualhost that will listen on a specified port; this is ugly but it
will at least succeed (note that I could remove the guards on proxying
since we're using a different port).

See https://github.com/facebook/hhvm/issues/3379

Change-Id: Ibf0b86b52e26fc68012969fe9797b370227cec15
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>