There's no such thing as a "family pack" license for OS X Server. Server has to be the OS *inside* the VM.
Given the cost of OS X Server licenses ($499 a pop) for something like testing web browsers you're almost certainly better off mounting a shelf in a data center rack, covering it with the cheapest Mac Minis you can buy ($599), and enabling VNC on them all than you are buying an Xserve+virtualizer+OS X Server licenses. Starting with the hardware, an 8-core Xserve with 24GB of RAM will put you $4600 bucks in the hole. VMware Fusion still doesn't seem to *explicitly* "support " OS X guests, so let's tack on $1250 for a Parallels Server license (which does explicitly support OS X Server guests.). Simple math at this point determines that you'll have to be able to run 58 simultaneous OS X Server guests to match the price-per-session of OS X minis. Certainly when you factor in the network ports and additional rack space/power for the minis it might change things somewhat, but since you're really *not* going to be able to run 58 sessions at once on a single Xserve the equation is still pretty bad. Might as well just buy a Mini for every developer and call it a day.