bootorder: don't print empty 'order=' property

bootorder: don't print empty 'order=' property

Specifying 'boot: order=' was intended to be used for an empty bootorder
(i.e. no boot devices), but as it turns out our format parser doesn't
like empty '-list' properties if they are nested in a subformat.

Fixing this in JSONSchema sounds like a risky move, so instead just
write 'boot: ' (without 'order=') to indicate an empty bootorder. The
rest of the code handles it just fine, as this was valid before too.

Incidentally also fixes a bug where you couldn't create a new VM without
any disks if no explicit 'boot' property was specified (i.e. a simple
'qm create 100' without any parameters would fail).

Reported-by: Dominic Jäger <d.jaeger@proxmox.com>
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>