Virt-backup

Fixes 51 where the clean command could fail if the json definition had an unexpected field.

Added a `-c/--config` option to specify a custom config path.

Cleanups and fixes in the CLI helpers.

Bug fix

* Fix setting storage type when pivoting a disk, add a compatibility layer for old `pending_info`.

Bug fix

* Fix resetting storage type to a qcow2 after the backup of a shutdown domain. PR 44
* Match domains by UUID, fixing issues with shutdown domains. PR 43

Thanks a lot arkadiam for your contributions!

Bug fix

* Listing backups for specific groups now works. `virt-backup list testgroup` will list backups only for `testgroup`.
* Compares domains by using the libvirt ID instead of just object comparison.

Cleaning

* Remove the use of defusedlxml as the library is going to be deprecated. Use lxml directly instead, but remove unsafe options from the parser.
* Fix warnings with `arrow`.