add new 'boot' property format and introduce legacy conversion helpers
The format is unused in this commit, but will replace the current
string-based format of the 'boot' property. It is included since the
parameter of bootorder_from_legacy follows it.
Two helper methods are introduced:
* bootorder_from_legacy: Parses the legacy format into a hash closer to
what the new format represents
* get_default_bootdevices: Encapsulates the legacy default behaviour if
nothing is specified in the boot order
resolve_first_disk is simplified and gets a new $cdrom parameter to
control the behaviour of excluding CD-ROMs or instead searching for only
them.
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>