virtio_net: Sync rss config to device when virtnet_probe

During virtnet_probe, default rss configuration is initialized, but was
not committed to the device. This patch fix this by sending rss command
after device ready in virtnet_probe. Otherwise, the actual rss
configuration used by device can be different with that read by user
from driver, which may confuse the user.

If the command committing fails, driver rss will be disabled.

Fixes: c7114b12 ("drivers/net/virtio_net: Added basic RSS support.")
Signed-off-by: Philo Lu <lulie@linux.alibaba.com>
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Joe Damato <jdamato@fastly.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>