Description of problem:
Certain QEMU does not support live snapshotting.
VDSM, through libvirt, report this capability to Engine, which reports back to the user in the Host tab, and inhibit the functionality if QEMU lacks it.

If live snapshot reporting is missing, Engine assumes it is supported and works as usual for backward compatibility.

Unfortunately, one sequence of events is not covered
- VDSM reports live snapshot as disabled
- libvirt is updated (https://bugzilla.redhat.com/show_bug.cgi?id=1149667)
  and stops to report the capability. This makes VDSM stop reporting as well
- Engine no longer updates its tables, so it is stuck with old value
- live snapshot is gone without good reason

Mitigation:
apply libvirt update before to add any host

Workaround:
After libvirt is updated, with Engine shutoff,
update DB table

update vds_dynamic set is_live_snapshot_support='true'  -- for all affected hosts

Version-Release number of selected component (if applicable):
3.5.0

How reproducible:
100% (estimated)

Steps to Reproduce:
see description above

Actual results:
Live snapshot support still reported as disabled

Expected results:
Live snapshot support available again