segfault when kvm modules are not loaded

Hi there,

This is probably minor, but I'm getting the following segfault when
trying to run qemu-kvm w/o kvm modules loaded:

"""
sudo ./qemu-kvm -hda fedora-11-kratos-i386.img -m 1G -snapshot open /dev/kvm: No such file or directory
Could not initialize KVM, will disable KVM support Segmentation fault (core dumped)
"""

I've debugged a little and found that:

1. Problem was introduced by 79c788

2. Segfault happens in exec.c:901, because the second call to tb_alloc() fails