qemu-ga: build it even if !system

Move qemu-ga build check out of if softmmu.. into its own section.
We want to build qemu-ga for _guest_ even if system build isn't
done.  It is controlled separately using --enable-guest-agent.
Additionally, give error message if guest agent is requested but
not supported.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
