QemuOpts: change opt->name|str from (const char *) to (char *)

QemuOpts: change opt->name|str from (const char *) to (char *)

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: 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>