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
Based on "backup compatibility" tests, which manually assemble the
backup. This is because we don't have access to actual backup creation
code here.
QubesOS/qubes-issues#1214
