virtio: Fix no interrupt when not creating msi controller

For ARM virt machine, if we use virt-2.7 which will not create ITS node,
the virtio-net can not recieve interrupts so it can't get ip address
through dhcp.
This fixes commit 83d768b5(virtio: set ISR on dataplane notifications).

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>