net/virtio-user: support Rx interrupt

net/virtio-user: support Rx interrupt

For rxq interrupt, the device (backend driver) will notify driver
through callfd. Each virtqueue has a callfd. To keep compatible
with the existing framework, we will give these callfds to
interrupt thread for listening for interrupts.

Before that, we need to allocate intr_handle, and fill callfds
into it so that driver can use it to set up rxq interrupt mode.

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>