We used to walk the backing file chain at least twice per disk,
once to set up cgroup device whitelisting, and once to set up
security labeling.  Rather than walk the chain every iteration,
which possibly includes calls to fork() in order to open root-squashed
NFS files, we can exploit the cache of the previous patch.

* src/conf/domain_conf.h (virDomainDiskDefForeachPath): Alter
signature.
* src/conf/domain_conf.c (virDomainDiskDefForeachPath): Require caller
to supply backing chain via disk, if recursion is desired.
* src/security/security_dac.c
(virSecurityDACSetSecurityImageLabel): Adjust caller.
* src/security/security_selinux.c
(virSecuritySELinuxSetSecurityImageLabel): Likewise.
* src/security/virt-aa-helper.c (get_files): Likewise.
* src/qemu/qemu_cgroup.c (qemuSetupDiskCgroup)
(qemuTeardownDiskCgroup): Likewise.
(qemuSetupCgroup): Pre-populate chain.