kvm tools: Boot to host /bin/sh via 9p by default

This patch updates 'kvm run' to boot to host filesystem via 9p '/bin/sh' by
default:

  $ ./kvm run
    # kvm run -k ../../arch/x86/boot/bzImage -m 320 -c 2 --name guest-3462
  [    0.000000] Linux version 3.1.0-rc1+ (penberg@tiger) (gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5) ) #7 SMP PREEMPT Tue Aug 9 16:39:20 EEST 2011
  [    0.000000] Command line: notsc noapic noacpi pci=conf1 reboot=k panic=1 console=ttyS0 earlyprintk=serial  init=/bin/sh  root=/dev/vda rw  root=/dev/root rootflags=rw,trans=virtio,version=9p2000.u rootfstype=9p

  [snip]

  [    1.803261] VFS: Mounted root (9p filesystem) on device 0:13.
  [    1.805153] devtmpfs: mounted
  [    1.808353] Freeing unused kernel memory: 924k freed
  [    1.810592] Write protecting the kernel read-only data: 12288k
  [    1.816268] Freeing unused kernel memory: 632k freed
  [    1.826030] Freeing unused kernel memory: 1448k freed
  sh: cannot set terminal process group (-1): Inappropriate ioctl for device
  sh: no job control in this shell
  sh-4.1#

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>