virtio: disable ioeventfd as early as possible

Avoid "tricking" virtio-blk-dataplane into thinking that ioeventfd will be
available when it is not.  This bug has always been there, but it will break
TCG+ioeventfd=on once the dataplane code will be always used when ioeventfd=on.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>