virtio-ccw: fix build breakage on windows

event_notifier_get_fd() is not available on windows hosts. Fix this by
moving the calls to event_notifier_get_fd() to the kvm code.

Reported-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>