CLOUDSTACK-4002: We should use backup_snap_id in snapshots table to
construct snapshot install_path in snapshot_store_ref for pre-4.2.0
snapshot.
Summary: Quiet erroneous stopVM failures on kvm

Detail: Cloudstack tries to stop VMs all the time, for all sorts of reasons,
but usually just to get into a known state.  Libvirt throws an exception of
'Domain not found' when attempting to stop a VM that doesn't exist. This causes
problems for troubleshooting real issues. Domain not found should equate
to success if trying to stop.

BUG-ID: CLOUDSTACK-4011
Bugfix-for: 4.2
Signed-off-by: Marcus Sorensen <marcus@betterservers.com> 1375825281 -0600