api: add handling for new boot order format

api: add handling for new boot order format

The API is updated to handle the deprecation correctly, i.e. when
updating the 'order' attribute, the old 'legacy' (default_key) values
are removed (would now be ignored anyway).

When removing a device that is in the bootorder list, it will be removed
from the aforementioned. Note that non-existing devices in the list will
not cause an error - they will simply be ignored - but it's still nice
to not have them in there.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>