doc: note a pitfall on vhost reconnect feature

doc: note a pitfall on vhost reconnect feature

The vhost feature negotiation only happens at virtio reset stage, say
when a virtio-net device is firstly initiated, or when DPDK virtio PMD
initiates. That means, if vhost APP restarts after the negotiation and
reconnects, the feature negotiation process will not be triggered again,
meaning the info is lost. To make reconnect work, QEMU simply saves
the negotiated features before the restart and restores it afterwards.

Therefore, the vhost supported features must be exactly the same before
and after the restart. For example, if TSO is disabled and then enabled,
nothing will work and undefined issues might happen.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>