qemuDomainCleanupRun: Actually run cleanup callbacks in reverse order

qemuDomainCleanupRun: Actually run cleanup callbacks in reverse order

We have a framework to register cleanup callbacks that are run
when a domain is shut down. The idea is to run callbacks in
reverse order than they were registered. However, looking at the
code this is not the case. Fortunately, this framework is used to
register a single callback and a single callback only -
qemuMigrationDstPrepareCleanup() - therefore there was no problem
just yet.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>