Use append to a truncated file to retain owner (#1677)

Change the default behaviour when logging output from the console to a
file to ensure the file is pre-created and unless append is explicitly
set to true in the source config, truncate the file. Combined with
having the domain template default to enabling append, this causes
libvirt/qemu to not attempt to create the file and thus retains the
owner/group and permissions of the original file.

Relates-to: #1385

spec/unit/templates/domain_all_settings.xml

@@ -105,7 +105,7 @@
       <target dev='fda' bus='fdc'/>
     </disk>
     <serial type='file'>
-      <source path='/var/log/vm_consoles/machine.log'/>
+      <source path='/var/log/vm_consoles/machine.log' append='on'/>
       <target port='0'/>
     </serial>
     <console type='file'>