core: add support to update running vm

Allow user update running vm, by saving configuration in a special
next_run snapshot (if needed).
and restore this configuration once the vm is down.

Feature page: http://www.ovirt.org/Features/Edit_Running_VM
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1072313
Change-Id: Ie433352852f53f62e58349ce85475ee89e37ce89
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>