qemuhotplugtest: Don't plug a SCSI disk at unit 7
    
    Unit number 7 is kind of special. It's reserved for SCSI
    controller. The comment in virDomainSCSIDriveAddressIsUsed()
    summarizes that pretty nicely. Libvirt would never generate
    such address.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
    Reviewed-by: Jim Fehlig <jfehlig@suse.com>