Ensure that instances_path is not world readable

It could be a slight security problem to let every user on the machine
see our image files. With this patch the compute driver will take care
of changing the mode of the instances path so it is only readable by the
owner and set an adequate umask (0007) when creating the files for each
instance (disks, console log, ...).

This change also sets the executable flag for others to instances_path,
which is the only requirement so libvirt can access the contained files
for the instances.

Another issue with libvirt is that it sets the ownership of the images
to root after destroying the instance. This prevents further access from
nova if we have set previously a stric file mode. This patch will change
the ownership of those files to the nova user once the instance is destroyed.

Fixes bug 1129748

Change-Id: Ib290f8df20a9e29ed98faec221f607b685e26da4