s390-bios: Use control unit type to determine boot method

s390-bios: Use control unit type to determine boot method

The boot method is different depending on which device type we are
booting from. Let's examine the control unit type to determine if we're
a virtio device. We'll eventually add a case to check for a real dasd device
here as well.

Since we have to call enable_subchannel() in main now, might as well
remove that call from virtio.c : run_ccw(). This requires adding some
additional enable_subchannel calls to not break calls to
virtio_is_supported().

Signed-off-by: Jason J. Herne <jjherne@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1554388475-18329-14-git-send-email-jjherne@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>