The issue seems to be that the disk provided during deploy was not in a pristine state; this is required by the ceph-osd charm to use it as a backing device.
If your virtual MAAS has cinder or any other volume handler, you can use the 'osd-devices' item under 'storage' to specify the number of volumes and their size. Otherwise, for device paths, you have to make sure that they are pristine (i.e: have been zapped, as you did).
The issue seems to be that the disk provided during deploy was not in a pristine state; this is required by the ceph-osd charm to use it as a backing device.
If your virtual MAAS has cinder or any other volume handler, you can use the 'osd-devices' item under 'storage' to specify the number of volumes and their size. Otherwise, for device paths, you have to make sure that they are pristine (i.e: have been zapped, as you did).