pc-bios/s390-ccw: Scan through all devices if no boot device specified

pc-bios/s390-ccw: Scan through all devices if no boot device specified

If no boot device has been specified (via "bootindex=..."), the s390-ccw
bios scans through all devices to find a bootable device. But so far, it
stops at the very first block device (including virtio-scsi controllers
without attached devices) that it finds, no matter whether it is bootable
or not. That leads to some weird situatation where it is e.g. possible
to boot via:

 qemu-system-s390x -hda /path/to/disk.qcow2

but not if there is e.g. a virtio-scsi controller specified before:

 qemu-system-s390x -device virtio-scsi -hda /path/to/disk.qcow2

While using "bootindex=..." is clearly the preferred way of booting
on s390x, we still can make the life for the users at least a little
bit easier if we look at all available devices to find a bootable one.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1846975
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200806105349.632-7-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>