security: Do not restore labels on device tree binary
    
    A device tree binary file specified by /domain/os/dtb element is a
    read-only resource similar to kernel and initrd files. We shouldn't
    restore its label when destroying a domain to avoid breaking other
    domains configure with the same device tree.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>