vl: Tighten parsing of -machine option phandle_start

Make it QEMU_OPT_NUMBER, so it gets parsed by generic code, which
actually bothers to check for errors, rather than its user, which
doesn't.

Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Alexander Graf <agraf@suse.de>
Message-id: 1372943363-24081-8-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>