virtio-pci: introduce pio notification capability for modern device

We used to use mmio for notification. This could be slow on some arch
(e.g on x86 without EPT). So this patch introduces pio bar and a pio
notification cap for modern device. This ability is enabled through
property "modern-pio-notify" for virtio pci devices and was disabled
by default. Management can enable when it thinks it was needed.

Benchmarks shows almost no obvious difference compared to legacy
device on machines without ept. Thanks Wenli Quan <wquan@redhat.com>
for the benchmarking.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>