ceph-volume: fix nvme preformat wiping VG on osds-per-device > 1

prepare_data_device() was calling nvme_utils.preformat() unconditionally
for every OSD slot. With --osds-per-device N, this means the second osd
reformats the device after the first OSD had already created its vg
on it which makes vgcreate fail with "device has a signature".

this commit fixes this issue by checking api.get_device_vgs() before
preformatting: if a ceph VG already exists on the device, skip the format.
This mirrors the logic create_lv() already uses to reuse an existing VG.

Fixes: https://tracker.ceph.com/issues/78615

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>