Add xtrabackup to the guest-image for backup / restore

We need to bake xtrabackup into the guest image so that we
can run tests for backup / restore scenarios.

Fixes Bug 1175847

Change-Id: I0b5f6188d018cd98dc1b49b69a06de3c12c3e4b9
Merge "Add xtrabackup to the guest-image for backup / restore"