New qvm-backup tool can either use pre-existing backup profile
(--profile), or - when running in dom0 - can create new one based on
used options (--save-profile).
This commit add a tool itself, update its man page, and add tests for
it.
FixesQubesOS/qubes-issues#2931
Most of them are moved from core-admin repository, even if tool is not
implemented yet. Some are generated using skel-manpage.py script.
This way we have complete man pages list, and sphinx will properly
warn/error about inconsistencies.
