qemu: Fix startupPolicy regression

qemu: Fix startupPolicy regression

Commit 82d5fe543720da6d83c1d6bfa1c347d7d9fda278

    qemu: check backing chains even when cgroup is omitted

added backing file checks just before the code that removes optional
disks if they are not present. However, the backing chain code fails in
case the disk file does not exist, which makes qemuProcessStart fail
regardless on configured startupPolicy.

Note that startupPolicy implementation is still wrong after this patch
since it only check the first file in a possible chain. It should rather
check the complete backing chain. But this is an existing limitation
that can be solved later. After all, startupPolicy is most useful for
CDROM images and they won't make use of backing files in most cases.