Applications crash when using a visual with 24 bits per pixel

Ubuntu Unity2D (a Qt/QML shell) segfaults when run in the KVM machine emulator using the Cirrus driver, which sets a visual having 24 bits per pixel and depth 24.
For more information, please see:
https://bugs.launchpad.net/ubuntu/+source/unity-2d/+bug/804784
https://bugs.launchpad.net/unity-2d/+bug/805587

I could reproduce the same crashes in VirtualBox, having such a xorg.conf:

==========
Section "Serverflags"
Option "Pixmap" "24"
EndSection

Section "Screen"
Identifier "Default Screen"
Device "VirtualBox graphics card"
Monitor "Generic Monitor"
DefaultDepth 24
DefaultFbBpp 24
SubSection "Display"
Depth 24
FbBpp 24
Modes "1024x768" "800x600" "640x480"
EndSubSection
EndSection
==========