vhost-user: introduce protocol feature for skip drain on GET_VRING_BASE

vhost-user: introduce protocol feature for skip drain on GET_VRING_BASE

Add vhost-user protocol feature
VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT

Now on GET_VRING_BASE this feature can control whether to wait for
in-flight requests to complete or not.
Also we have to validate that this feature will be enabled only when
qemu and back-end supports in-flight buffer and in-flight migration

It will be helpfull in future for in-flight requests migration in
vhost-user devices.

Update docs, add ref to label for inflight-io-tracking

Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260115081103.655749-3-dtalexundeer@yandex-team.ru>