running ubuntu in a virtual machine by qemu-system

I wanted to start an ubuntu virtual machine as follows in Knoppix 7.2.0DVD (booted with 'fromhd'):
root# qemu-system-x86_64 -cdrom ubuntu-12.04.3-dvd-amd64.iso

The iso image was read OK, with the ubuntu boot screen appearing in the vga display of the emulator. However the booting failed with the following message:
[ 29.044201] BUG: soft lockup - CPU#0 stuck for 23s! [swapper/0:1]
[ 53.044186] BUG: soft lockup - CPU#0 stuck for 22s! [swapper/0:1]
[ 89.044214] BUG: soft lockup - CPU#0 stuck for 23s! [swapper/0:1]

When the iso image is burned to a DVD, the ubuntu system boots up without a problem from the disk. And "qemu-system-x86_64" starts a Knoppix VM from iso without problems. So it appears that the ubuntu iso and the qemu behaves as expected individually.

Although this problem may reside well outside the domain of Knoppix per se, I hope I may receive advice on where to go from here. Thanks in advance for any help.