vbucket-test: fixed path handling

Prepend a '/' to our desired destination

Change-Id: I7611d18d5ba3fecf7916d2addd7aa7b7e7373de1
Reviewed-on: http://review.couchbase.org/38085
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>