pc-bios/s390-ccw: Split virtio-scsi code from virtio_blk_setup_device()

The next patch is going to add more virtio-block specific code to
virtio_blk_setup_device(), and if the virtio-scsi code is also in
there, this is more cumbersome. And the calling function virtio_setup()
in main.c looks at the device type already anyway, so it's more
logical to separate the virtio-scsi stuff into a new function in
virtio-scsi.c instead.

Message-Id: <20220704111903.62400-10-thuth@redhat.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>