OCaml tools: Use Common_gettext and Common_utils modules.

Share these modules across all three tools virt-resize, virt-sparsify
and virt-sysprep.

This is mostly code motion.
fish: Better tab completion for guestfish.
Add --long-options option to most tools.

For example:

$ guestfish --long-options
--add
--cmd-help
--connect
--csh
--domain
--echo-keys
[etc.]

The idea of this is to make it easier to write a bash completion
script that accurately expands --<TAB> options for each command.
bash completion: Extend this script to work with other virt tools.
Version 1.21.23.