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, 8 insertions, 0 deletions

diff --git a/appliance/init b/appliance/init
index 8b97590a..545232ce 100755
--- a/appliance/init
+++ b/appliance/init
@@ -65,7 +65,15 @@ if ! grep -sq guestfs_rescue=1 /proc/cmdline; then
   exec guestfsd -f
 fi
 
+# Use appliance in rescue mode, also used by the virt-rescue command.
 TERM=linux ;# XXX library should pass this from library's environment
 PS1='><rescue> '
 export TERM PS1
+echo
+echo "Welcome to virt-rescue, the libguestfs rescue shell."
+echo
+echo "Note: The contents of / are the rescue appliance."
+echo "You have to mount the guest's partitions under /sysroot"
+echo "before you can examine them."
+echo
 exec bash -i