xenapi: script to rotate the guest logs

Xen will produce the guest logs in a specified directory.

This scripts configures the above logging as expected
by the xapi plugin nova uses to read the logs.
In addition it ensures that any log that grows bigger
than 1MB will be truncated down to 5KB.

Ensuring the log directory is inside a loop back device
of a restricted size, and running this script every
minute using cron, will stop guests from using up
too much Dom0 disk space, and taking out the hypervisor

Part of blueprint xenapi-server-log
Change-Id: I4caa82f4d0620d924e37e3b605cf62b4d5b73570
Merge "xenapi: script to rotate the guest logs"