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.

Change-Id: I5449eee954bd6096b6f2e423a75a8cac3e5bcc3b
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>