expose QemuOpt and QemuOpts struct definitions to interested parties

The only clients should be the existent "qemu-option.c", and the upcoming
"qapi/opts-visitor.c".

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>