Yes, for sure we need to do a check.

However, this would involve - I think - some non-trivial modifications to net/netmap.c, because without NS_MOREFRAG you cannot split a packet over more "netmap slots/descriptors" (both tx and rx side)

Therefore I would ask (manly Luigi, since netmap is in-tree in FreeBSD): Wouldn't it be better to force --disable-netmap when we realize that NETMAP_API (version number for the netmap API) is less than the value required by QEMU? This can be done in ./configure. In this way we keep the QEMU code cleaner.