s390-bios: Use control unit type to find bootable devices

When the user does not specify which device to boot from then we end
up guessing. Instead of simply grabbing the first available device let's
be a little bit smarter and only choose devices that might be bootable
like disk, and not console devices.

Signed-off-by: Jason J. Herne <jjherne@linux.ibm.com>
Message-Id: <1554388475-18329-17-git-send-email-jjherne@linux.ibm.com>
[thuth: Added fix for virtio_is_supported() not being called anymore]
Signed-off-by: Thomas Huth <thuth@redhat.com>