fix #2173: use qemu-img to check cloudinit disk existence

fix #2173: use qemu-img to check cloudinit disk existence

use file_size_info to check for existence of cloudinit disk instead of
'-e'. It uses `qemu-img info` to get some file info, which can handle
rbd, and various other paths for volumes not exposed as normal file
or not mapped, yet.

this addresses a problem with rbd where the path returned available
is not checkable with '-e'.

Any size > 0 is interpreted as the image existing.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>