s390x/css: disabled subchannels cannot be status pending
    
    The 3270 code will try to post an attention interrupt when the
    3270 emulator (e.g. x3270) attaches. If the guest has not yet
    enabled the subchannel for the 3270 device, we will present a spurious
    cc 1 (status pending) when it uses msch on it later on, e.g. when
    trying to enable the subchannel.
    
    To fix this, just don't do anything in css_conditional_io_interrupt()
    if the subchannel is not enabled. The 3270 code will work fine with
    that, and the other user of this function (virtio-ccw) never
    attempts to post an interrupt for a disabled device to begin with.
    
    CC: qemu-stable@nongnu.org
    Reported-by: Thomas Huth <thuth@redhat.com>
    Tested-by: Thomas Huth <thuth@redhat.com>
    Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
    Acked-by: Halil Pasic <pasic@linux.ibm.com>
    Reviewed-by: David Hildenbrand <david@redhat.com>
    Signed-off-by: Cornelia Huck <cohuck@redhat.com>
    (cherry picked from commit 6e9c893ecd00afd5344c35d0d0ded50eaa0938f6)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>