vhost-net-user: add stubs for when no virtio-net device is present

vhost-net-user: add stubs for when no virtio-net device is present

hw/net/vhost_net.c needs functions that are declared in net/vhost-user.c: the
vhost-user code is always compiled into QEMU, only the constructor
net_init_vhost_user is unreachable.  Also, net/vhost-user.c needs functions
declared in hw/virtio/vhost-stub.c even if no virtio device exists.

Break this dependency.  First, add a minimal version of net/vhost-user.c,
with no functionality and no dependency on vhost code.  Second, #ifdef out
the calls back to net/vhost-user.c from hw/net/vhost_net.c.

While at it, this patch fixes the CONFIG_VHOST_NET_USE*D* typo.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <1543851204-41186-3-git-send-email-pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1550165756-21617-3-git-send-email-pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>