launch: libvirt: When root, set socket permissions to 0660 (vs 0775).

Since qemu connects to both, there was no point at all in having these
sockets be readable by other, nor executable by anyone.

The sockets should now end up as:

  srw-rw---- root.qemu
launch: libvirt: Rename 'is_root' as 'current_proc_is_root', and documentation.
launch: libvirt: When debugging, print permissions of appliance and sockets.

When debugging is enabled, this prints out the permissions (ie.
ls -laZ) of the appliance and sockets directories.  This should be
helpful for debugging RHBZ#913774.