qemu_opt_del() already assumes that all QemuOpt instances contain
malloc'd name and value; but it had to cast away const because
opts_start_struct() was doing its own thing and using static storage
instead.  By using the correct type and malloced strings everywhere, the
usage of this struct becomes clearer.
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NLeandro Dorileo <l@dorileo.org>
Signed-off-by: NChunyan Liu <cyliu@suse.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>