Black screen with Unity8 snap session in zesty VM [This application failed to start because it could not find or load the Qt platform plugin "mirserver"]

[Richard Huddie]

Using up to date zesty vm and unity8-session snap installed, log into the unity8-snap session. A black screen is displayed and the session does not start, even after a long wait period.

unity8 log shows:
[2017-03-06:11:53:03.437] This application failed to start because it could not find or load the Qt platform plugin "mirserver"
in "".
Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, mirserver, ubuntumirclient, xcb.
Reinstalling the application may fix this problem.

I've tried unity8-session snap 457 from edge and 460 from beta, but both have the same problem.

snap environment:
Name Version Rev Developer Notes
address-book-app 0.2+17.04.20161219-0ubuntu1 25 canonical devmode
camera-app 3.0.0+17.04.20170106-0ubuntu1 13 canonical devmode
core 16-2 1337 canonical -
gallery-app 0.0.67+17.04.20161213-0ubuntu1 23 canonical devmode
mir-libs 0.26.1 25 canonical -
ubuntu-app-platform 1 34 canonical -
ubuntu-calculator-app 2.3.1 37 ubuntucoredev devmode
ubuntu-calendar-app 0.1.1 24 canonical devmode
ubuntu-clock-app 3.8.480 34 ubuntucoredev devmode
ubuntu-filemanager-app 0.4 16 canonical devmode
ubuntu-terminal-app 0.11 51 canonical devmode
unity8-session 16.04 460 canonical devmode
webbrowser-app 20170303~staging 13 canonical devmode