You would do good to mirror everything that oVirt is using, especially if you want to
install/rebuild while remaining offline.
The 1.1 GB file you mention is the oVirt appliance initial machine image, which
unfortunately seems to get explicitly deleted from time to time, most likely the official
clean-up scripts I use, whenever a deployment failed and I want to restart from a clean
sheet.
If you're operating disconnected, security bugs won't scare you, which is where
most of the updates are coming from. A good frozen CentOS7 repo state can last a long
time, but sometimes even those are glitchy so you need to test thoroughly before going
completely offline.
oVirt 4.3 is *very* stable (nothing done any more), but not free of bugs. You may be lucky
and in an offline mode the combination may be good... until you want online again or add
hardware too novel.
It's how I operate currently with most of my oVirt installations: CentOS7.latest and
oVirt-4.3.last-with-bugs, because I want to test stuff in the VMs not underneath the
hypervisor (and I am mostly using recycled producation hardware to support a lab).