docs: Move virtio-net-failover.rst into the system manual

The virtio-net-failover documentation is currently orphan and
not included in any manual; move it into the system manual,
immediately following the general network emulation section.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

docs/system/index.rst

@@ -21,6 +21,7 @@ Contents:
    monitor
    images
    net
+   virtio-net-failover
    usb
    ivshmem
    linuxboot