QemuOpts: add conversion between QEMUOptionParameter to QemuOpts

QemuOpts: add conversion between QEMUOptionParameter to QemuOpts

Add two temp conversion functions between QEMUOptionParameter to QemuOpts,
so that next patch can use it. It will simplify later patch for easier
review. And will be finally removed after all backend drivers switch to
QemuOpts.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Leandro Dorileo <l@dorileo.org>
Signed-off-by: Chunyan Liu <cyliu@suse.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>