qemu: Prepare storage backing chain traversal code for FD passed images

qemu: Prepare storage backing chain traversal code for FD passed images

We assume that FD passed images already exist so all existance checks
are skipped.

For the case that a FD-passed image is passed without a terminated
backing chain (thus forcing us to detect) we attempt to read the header
from the FD.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>