backup: prepare: fix format detection for disks without storage ID

backup: prepare: fix format detection for disks without storage ID

which is the case for passed-through disks. The qemu_img_format()
function cannot correctly handle those, and it's not safe to assume
they are raw (it's most likely, but not guaranteed), so just use the
storage method for the format like it was done before commit
efa3aa24 ("avoid list context for volume_size_info calls"). This will
use 'qemu-img info' to get the actual format.

Reported in the community forum:
https://forum.proxmox.com/threads/124794/
https://forum.proxmox.com/threads/124823/
https://forum.proxmox.com/threads/124818/

Fixes: efa3aa24 ("avoid list context for volume_size_info calls")
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Tested-by: Friedrich Weber <f.weber@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>