ewaol-distro: Reduce installed file permissions for 'others' where appropriate

For installed files which don't require 'others' to have 'read' permissions,
this patch reduces the permissions to be 0640.

Files changed:

  * Guest VM disk and kernel images
  * Xen Guest VM definition config files
  * Systemd override configuration for kernel printk verbosity

Issue-Id: SCM-4394
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: Iafd7d15c4c4b54709f2f158bd3427dae1a090340