Diskmanage: add append_partition sub

Diskmanage: add append_partition sub

we will use this for adding a partition to a disk when using a device
for ceph osd db/wal which already has partitions on it

first we search for the highest partition number, then add the partition
and search for the resulting device (we cannot assume to simply
append the number, e.g. from /dev/nvme0n1 we get /dev/nvme0n1pX)

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