vhost: fix silent queue enabling with legacy guests

vhost: fix silent queue enabling with legacy guests

vhost should notify the application in case of all vring state changes.

In general, application should not care about negotiation of
VHOST_USER_F_PROTOCOL_FEATURES. Protocol details like this should
be hidden by the vhost library.

With this patch applications like OVS will be able to assume that
all vrings disabled by default and only process 'vring_state_changed'
events.

Fixes: 321203a54ba7 ("vhost: enable rings at the right time")
Cc: stable@dpdk.org
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>