net/virtio-user: send kick notify backend on init

Acccording to the vhost-user spec [0], client must start ring
upon receiving a kick (that is, detecting that file descriptor
is reachable) on the descriptor specified by VHOST_USER_SET_VRING_KICK.

The code sends a kick to the rx queue. It is missing sending a
kick for the tx queue. This patch is to add the missing code to
comply with the spec.

[0]: https://fossies.org/linux/qemu/docs/specs/vhost-user.txt

Signed-off-by: Steven Luong <sluong@cisco.com>
Acked-by: Yuanhan Liu <yliu@fridaylinux.org>