v2v: Add 'exit' choice to --root ask.

If the user does ^C then this leaves temporary overlay files around
(possibly a bug?).  Offer an 'exit' choice to the user which exits
cleanly.

The new message looks like this:

  Dual- or multi-boot operating system detected.  Choose the root filesystem
  that contains the main operating system from the list below:

   [1] /dev/sda3 (Fedora release 20 (Heisenbug))
   [2] /dev/sdb3

  Enter a number between 1 and 2, or 'exit': exit