Add exception for missing kvm-img and qemu-img

[gdm85]

Trying to build on a system without kvm-img and qemu-img, the following happens:

2014-05-30 15:49:59,731 INFO : Mounting target filesystems
2014-05-30 15:49:59,732 INFO : Creating disk image: "/tmp/tmp88bqMh" of size: 11264MB
2014-05-30 15:49:59,734 INFO : Cleaning up
2014-05-30 15:49:59,734 ERROR : Couldn't find the program 'None' on your system
Traceback (most recent call last):
  File "/usr/local/bin/vmbuilder", line 24, in <module>
    cli.main()
  File "/usr/local/lib/python2.7/dist-packages/VMBuilder/contrib/cli.py", line 228, in main
    hypervisor.install_os()
  File "/usr/local/lib/python2.7/dist-packages/VMBuilder/hypervisor.py", line 65, in install_os
    self.call_hooks('mount_partitions', self.chroot_dir)
  File "/usr/local/lib/python2.7/dist-packages/VMBuilder/distro.py", line 67, in call_hooks
    call_hooks(self, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/VMBuilder/util.py", line 170, in call_hooks
    getattr(context, func)(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/VMBuilder/hypervisor.py", line 88, in mount_partitions
    disk.create()
  File "/usr/local/lib/python2.7/dist-packages/VMBuilder/disk.py", line 102, in create
    run_cmd(qemu_img_path(), 'create', '-f', 'raw', self.filename, '%dM' % self.size)
  File "/usr/local/lib/python2.7/dist-packages/VMBuilder/util.py", line 100, in run_cmd
    raise VMBuilderUserError, "Couldn't find the program '%s' on your system" % (argv[0])
VMBuilder.exception.VMBuilderUserError: Couldn't find the program 'None' on your system

I propose merging the small patch in attachment, perhaps with correct exception type.