Fix live migration of xen instances (#1582)
Fix live migration of xen instances (#1582)

This PR fixes issue #1574. Config deletion has been unified (setting the config path first and deleting the file, if it exists and creating it) and an improved runtime check has been introduced.

The test used before just took the cpu usage in seconds into account, which lead to problems on our new cluster due to its quite beefy AMD cpus; just migrating an idle instance did not increase the number of cpu seconds above 0.
ganeti/ganeti,ganeti/ganeti,ganeti/ganeti