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>

configure

@@ -231,7 +231,7 @@ libusb=""
 usb_redir=""
 glx=""
 zlib="yes"
-guest_agent="yes"
+guest_agent=""
 want_tools="yes"
 libiscsi=""
 coroutine=""
@@ -3444,10 +3444,15 @@ if test "$softmmu" = yes ; then
       virtfs=no
     fi
   fi
+fi
+if [ "$guest_agent" != "no" ]; then
   if [ "$linux" = "yes" -o "$bsd" = "yes" -o "$solaris" = "yes" ] ; then
-    if [ "$guest_agent" = "yes" ]; then
       tools="qemu-ga\$(EXESUF) $tools"
-    fi
+      guest_agent=yes
+  elif [ "$guest_agent" != yes ]; then
+      guest_agent=no
+  else
+      error_exit "Guest agent is not supported on this platform"
   fi
 fi