Adding VHOST_USER_PROTOCOL_F_CONFIG_MEM_SLOTS to 5.1 release notes

[Raphael Norwitz]

Hi mst,

Looking at the current changelog
https://wiki.qemu.org/ChangeLog/5.1#virtio, I don't see any mention of
the VHOST_USER_PROTOCOL_F_CONFIG_MEM_SLOTS protocol feature. It is a
user visible change so shouldn't we add a note?

Thanks,
Raphael