core: update vms on update of instance type

when instance type values are updated, we need to update also all the
vms that are using this instance type.

also, added audit log for vm update that is done internaly (by system)
otherwise log will show "user <unknown>"

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1136372
Change-Id: Ic229a677bdd61a7f522548d9abf68476fbe219e7
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>