add new 'boot' property format and introduce legacy conversion helpers

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>