sparsify: Add --tmp prebuilt:file option.

This option allows oVirt to pass a prebuilt qcow2 file to use as the
temporary overlay.  The file must be qcow2, and must have indisk as a
backing file - the code does minimal checks to ensure this is correct.

Example usage:

  qemu-img create -f qcow2 -b indisk overlay.qcow2
  virt-sparsify indisk --tmp prebuilt:overlay.qcow2 outdisk

Note this only applies in copying mode.
docs: Remove useless "SHELL QUOTING" section from manual pages.
customize: Update copyright messages.

No functional change.
customize: Use Common_utils.error function instead of failwith.

This is just code refactoring.
sysprep: Standard exception handling, taken from virt-builder & virt-v2v.