Screen corruption under KVM

[MegaBrutal]

I was experimenting with Ubuntu MATE, and I found severe screen corruption whenever I run the image under a KVM virtual machine. I attached a screenshot. I don't think the problem is Ubuntu MATE specific. I also found it doesn't depend on the display manager. (It can be either LightDM or GDM.)

Reproduce instructions:
1. Boot up a KVM virtual machine with an Ubuntu Vivid Vervet image.
2. Put some stress on the display: what I used to do is to launch Firefox and htop in a terminal, then lock the screen, or let it lock by itself.
3. Wait a little, then unlock the computer.
4. You'll see letters missing and images corrupted, like on the attached screenshot.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: xorg 1:7.7+7ubuntu4
ProcVersionSignature: Ubuntu 3.19.0-10.10-generic 3.19.2
Uname: Linux 3.19.0-10-generic x86_64
ApportVersion: 2.16.2-0ubuntu5
Architecture: amd64
Date: Sat Mar 28 02:04:36 2015
InstallationDate: Installed on 2015-03-27 (0 days ago)
InstallationMedia: Ubuntu-MATE 15.04 "Vivid Vervet" - Beta amd64 (20150326)
ProcEnviron:
 LANGUAGE=hu
 TERM=xterm
 PATH=(custom, no user)
 LANG=hu_HU.UTF-8
 SHELL=/bin/bash
SourcePackage: xorg
UpgradeStatus: No upgrade log present (probably fresh install)