Add 'virt-rescue' command.

This command runs a "rescue appliance" against a virtual machine
or disk image.  This is useful for making ad-hoc interactive
changes to virtual machines.

  $ virt-rescue --ro /dev/vg_trick/F11x64

  Welcome to virt-rescue, the libguestfs rescue shell.

  Note: The contents of / are the rescue appliance.
  You have to mount the guest's partitions under /sysroot
  before you will be able to examine them.

  bash: cannot set terminal process group (-1): Inappropriate ioctl for device
  bash: no job control in this shell
  ><rescue> mount /dev/vg_f11x64/lv_root /sysroot
  EXT4-fs (dm-0): barriers enabled
  kjournald2 starting: pid 269, dev dm-0:8, commit interval 5 seconds
  EXT4-fs (dm-0): internal journal on dm-0:8
  EXT4-fs (dm-0): delayed allocation enabled
  EXT4-fs: file extents enabled
  EXT4-fs: mballoc enabled
  EXT4-fs (dm-0): mounted filesystem with ordered data mode
  ><rescue> ls /sysroot/
  bin   dev  home  lib64       media  opt   root  selinux  sys  usr
  boot  etc  lib   lost+found  mnt    proc  sbin  srv      tmp  var
  ><rescue> exit

1 files changed, 3 insertions, 0 deletions

diff --git a/HACKING b/HACKING
index 7156bf70..a051d8a9 100644
--- a/HACKING
+++ b/HACKING
@@ -122,6 +122,9 @@ python/
 regressions/
         Regression tests.
 
+rescue/
+	The virt-rescue tool.
+
 ruby/
         Ruby bindings.