qemu: Use new connection close callbacks API

qemu: Use new connection close callbacks API

The qemu driver uses connection close callbacks in more places requiring
more changes than other drivers, but luckily the changes are very
straightforward. The migration code was written in a way ensuring that
there's just one callback present so this can be preserved directly.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>