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>
Peter Maydell