Booting a VM from cdrom require attaching the device before VM startup,
which is possible only in persistent mode. But for qvm-start --cdrom
adding a cdrom only temporarily, use new update_persistence() function
to convert the assignment to temporary one.
FixesQubesOS/qubes-issues#3055
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.
