qemu: Cancel storage migration in parallel
    
    Instead of cancelling disk mirrors sequentially, let's just call
    block-job-cancel for all migrating disks and then wait until all
    disappear.
    
    In case we cancel disk mirrors at the end of successful migration we
    also need to check all block jobs completed successfully. Otherwise we
    have to abort the migration.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>