QemuOpts: add some functions

qemu_opt_foreach: loop over all QemuOpts entries.
qemu_opts_id: return QemuOpts id.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Message-Id: 
