The "git archive" feature creates tarballs which are missing all
submodule content. GitHub unhelpfully provides users with "Download"
links that claim to give them valid source release tarballs. These
GitHub archives will not be buildable as they are created by the
"git archive" feature and so are missing content. The user gets
unhelpful messages from make such as:

  fatal error: ui/input-keymap-atset1-to-qcode.c: No such file or directory

By adding a sanity check we can give users an informative message about
what they've done wrong.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20180418171151.5263-1-berrange@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>