qemu-option: Fix qemu_opts_find() for null id arguments

Crashes when the first list member has an ID.  Admittedly nonsensical
reproducer:

$ qemu-system-x86_64 -nodefaults -machine id=foo -machine ""

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1372943363-24081-2-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
