virtio-mmio: ioeventfd support

set_host_notifier and set_guest_notifiers supported by virtio-mmio now.
Most code copied from virtio-pci.

This makes it possible to use vhost-net with virtio-mmio,
improving performance by about 30%.

The kvm-arm does not yet support irqfd, need to fix the hard-coded part after
kvm-arm gets irqfd support.

Signed-off-by: Ying-Shiuan Pan <yingshiuan.pan@gmail.com>
Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>