Add optional pre-shutdown: shutdown/launch vm before migration. This
leads to storing persistent bitmap to the storage, which breaks
migration with dirty-bitmaps capability enabled and shared storage
until fixed by previous commit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20191125125229.13531-3-vsementsov@virtuozzo.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>