vhost: fail backend intialization early Don't initialize vhost backend if memslots number exceeds the supported limit. This prevents failures down the road when backend is actually started. [MST: rewrite commit log] Signed-off-by: Igor Mammedov <imammedo@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>