fix bootdisk_size for new bootorder config scheme

fix bootdisk_size for new bootorder config scheme

Previously, we ever only had a single boot *disk*, while possibly
having multiple cdroms/nics in the boot order

e.g. the config:

 boot: dnc
 bootdisk: scsi0
 ide0: media=cdrom,none
 scsi0: xxx
 net0: ...

would return the size of scsi0 even though it would first boot
from cdrom/network.

When editing the bootorder with such a legacy config, we
remove the 'bootdisk' property and replace the legacy notation
with an explicit order, but we only search the first disk
for the size now.

Restore that behaviour by iterating over all disks in the boot
order property string until we get one that is not a cdrom
and has a size.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>