util: add iterators for QemuOpts values

To iterate over all QemuOpts currently requires using a callback
function which is inconvenient for control flow. Add support for
using iterator functions more directly

  QemuOptsIter iter;
  QemuOpt *opt;

  qemu_opts_iter_init(&iter, opts, "repeated-key");
  while ((opt = qemu_opts_iter_next(&iter)) != NULL) {
      ....do something...
  }

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170203120649.15637-8-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>