qemu_hotplug: Check for duplicate drive addresses
    
    This tries to fix the same problem as f1d65853000 but it's doing
    so in a less invasive way.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
    Reviewed-by: Jim Fehlig <jfehlig@suse.com>