vhost-user: wait until backend init is completed

vhost-user: wait until backend init is completed

The chardev waits for an initial connection before starting qemu, and
vhost-user should wait for the backend negotiation to be completed
before starting qemu too.

vhost-user is started in the net_vhost_user_event callback, which is
synchronously called after the socket is connected. Use a
VhostUserState.started flag to indicate vhost-user init completed
successfully and qemu can be started.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>