Disable file injection by default

File injection in libvirt is problematic in havana on some systems.
When doing a live upgrade where the system suddenly gains libguestfs
support, a compute node may start using it, and thus start breaking.

Related master branch patch: I5015f2c351b1d680c205d7f9a5204febca490b91

Change-Id: Ic71baa25cda4b65c79c7c5f64bbb802774e489f4
Related-Bug: #1287354
Merge "Disable file injection by default" into stable/havana