spapr-vlan: flush queue whenever can_receive can go from false to true

When the guests adds buffers to receive queue, the network device
should flush its queue of pending packets. This is done with
qemu_flush_queued_packets.

This adds a call to qemu_flush_queued_packets() which wakes up the main
loop and let QEMU update the network device status which now is "can
receive". The patch basically does the same thing as e8b4c68 does.

Suggested-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>