OP: Just a few paragraphs down herein is links with relevant howto, doc, not terribly old, but I'm not aware enough (I've experience in a nearby but not exactly your neighborhood). If you read, you'll see.
Ah, but Slack lends itself to "roll up your sleeves" while having fun working (or is it learning) (or is it learning/creating) with respect to "what makes it tick" (especially I mean here as far as the creating of an app/method for a specified task or my own customization of Slack).
I have fun customizing is one of the reasons that I use Slack.
Is it that "the creative side" (of me in my case) gets to take expression -- seems, to some degree, true. (I think Slack lends itself to this as one of the characteristics or reasons to use Slack).
I logged in then searched here at linuxquestions at the slackware forum using "virtualization" for the keyword/search_term.
Next consists of a part of those hits (BTW some have howto and/or further doc on links/softwares/apps that you already shared in your post) and/or "one thing leads to another" ie "links" led off to other links etc. etc.
I've (acummings at next url/thread) dinked around with Qemu and Vmware workstation/Vmware Player
But I wasn't using it for server. I ran Win 2K in virtual machine on top of Slack (Slack is host, Win 2K is guest OS) (strange or maybe not strange desktop useage ie 2 (vastly) different OS's running at once on one hardware box). I could (but I won't) argue for that it's a server that serves up (for me to use) two different desktop OS's all at once in real time.
While in the qemu monitor, I saw therein the ability to make snapshot(s) of the virtual machine. Snapshot allows "return to exact state" (of where it was when snapshot had been taken). (I just keep a copy of my Win 2k virtual machine file (the file, the so called "image" file that) runs inside Qemu -- I haven't yet used "snapshot" feature)
Some virtual machine capable of "disallow write access" -- except only in a dedicated or safe area (maybe this be similar to "jail" here) and even then "only until this virt. mach. is turned off or rebooted" (said write access, anything written is discarded and erased upon reboot of this virt. mach.) IOW: "always boots up pristine" (this virt. mach.)
I'm unaware of which all of the different virt. mach. out there as to which of them have whatever of the different capabilities. I don't know if Qemu can be set (has such an opt or setting) to boot an image "always boots pristine".
Lots of different virtuals out there for a lot of different reasons/applications. It's worthy to explore/discover/investigate as to "what would best suit or some of the better fit(s) for what your specific need is.
I saw reasons of "it saves on disk space" and "it saves on memory/resources" (to run one or some of the apps that you mentioned rather than to have the additional overhead of running a second OS inside a virtual machine).
And I see and honor those reasons/points. I'm gung ho for the above reasons whenever the need is or can be met by those reasons/method(s).
I'm not at all trying to sway you to the overhead of running a second OS in a virtual machine.
But, much of what I've shared here, is merely where my experience has lied, until now. Until now.
(grabs some more around toits). Hmm, Xen and KVM already on the (soon to happen in the future) agenda. Or, will I experiment first with some of the virtual server apps that you mentioned and then do Xen and KVM.
So much to do, so little time. Writing is fun too.