core: automatically update vms on new version

When new version of a template is available,
or when stateless(\pool) vm goes to down,
we update the vm to the new version.

http://www.ovirt.org/index.php?title=Features/Template_Versions

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1037478
Change-Id: I5449eee954bd6096b6f2e423a75a8cac3e5bcc3b
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>