virt-manager hang after "mounting /sys filesystem"

So, when installing CentOS 5.2 (and possibly other distros) with virt-manager, console output disappears after the "mounting /sys filesystem" line.  This might only happen in low-memory conditions.  (The fact that anything less than 512 MB is considered low-memory for an _installer_, by the way, I find appalling.)

To work around this issue, install using the extra kernel argument "xencons=tty" (no quotes.)