Here is the summary of the situation.
Latest Virtual Box version available (Version 5.0.26 r108824) + latest guest additions installed.
On Virtual Box, Guest Windows 7 x64, MSVC2015, -opengl dynamic, with 3D acceleration enabled:
QWebEngine app + Desktop OpenGL (QT_OPENGL environment variable set to desktop) does not work, because of issues with the Virtual box OpenGL driver, not much we can do there.
QWebEngine app + ANGLE (QT_OPENGL to angle) seems to work fine.
QWebEngine app + Software rendering (QT_OPENGL to software) seems to work fine as well.
On Virtual Box, Guest Ubuntu 15.10 x64, with 3D acceleration enabled:
QWebEngine apps won't work, because of issues with the Virtual box OpenGL driver, not much we can do.
If 3D acceleration is disabled in VirtualBox, both on windows and linux the apps work fine, but are obviously slower.
No tests were done on VMWare, because there's no license available at the moment.
Previously before 5.6.x, on Windows + ANGLE there were issues because the Chromium part of the WebEngine used GPU acceleration as well as the Qt part, which led to multi-threading issues. Now the equivalent of --disable-gpu is sent to Chromium, using GPU acceleration only on the Qt side, thus ANGLE in VMs should work.