[tor-bugs] #19738 [Applications/Tor Browser]: Update check-prerequisites.sh

[Tor Bug Tracker & Wiki]

#19738: Update check-prerequisites.sh
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     Reporter:  gk                        |      Owner:  tbb-team
         Type:  task                      |     Status:  new
     Priority:  Medium                    |  Milestone:
    Component:  Applications/Tor Browser  |    Version:
     Severity:  Normal                    |   Keywords:  tbb-gitian
Actual Points:                            |  Parent ID:
       Points:                            |   Reviewer:
      Sponsor:                            |
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 Setting up a new build machine brought some bugs in our `check-
 prerequisites.sh` to light.

 1) On Debian I needed `libvirt-bin` as well
 2) `tor-browser-builder-4` is a thing now
 3) Ubuntu 16.04 with the vmbuilder (we need) packaged is available. We
 should take that into account and not point them to our work around.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19738>
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