Never mount partitions on a disk with the iso9660 filesystem (#1878784)
    
    Blivet doesn't recognize these partitions, so we mount the disk in stage2.
    However, it will fail if one of its partitions is already mounted, for example
    in stage1. Therefore, skip these partitions in the script that looks for a root
    image on devices and use the disk instead.
    
    The boot.iso has the following structure:
    
    NAME   TYPE  SIZE FSTYPE  LABEL
    sda    disk  8.8G iso9660 RHEL-8-3-0-BaseOS-x86_64
    |-sda1 part  8.8G iso9660 RHEL-8-3-0-BaseOS-x86_64
    `-sda2 part   10M vfat    ANACONDA
    
    And the following default boot options:
    
    inst.stage2=hd:LABEL=RHEL-8-3-0-BaseOS-x86_64 rd.live.check quiet
    
    Anaconda runs the anaconda-diskroot script for every device that matches the
    specification from the inst.stage2 option. In this example, it is sda and sda1,
    because both of them have the specified label RHEL-8-3-0-BaseOS-x86_64. With
    the fix, the script will detect that sda1 is a partition on a disk with the
    iso9660 filesystem and stop to process sda1. The script will succeed with sda.
    
    We should never rely on the order of the devices.
    
    Resolves: rhbz#1878784