qemu-option: qemu_opt_set_bool(): fix code duplication

It will set opt->str in qemu_opt_set_bool, without opt->str, there
will be some potential bugs.

These are uses of opt->str, and what happens when it isn't set:

* qemu_opt_get(): returns NULL, which means "not set".  Bug can bite
  when value isn't the default value.

* qemu_opt_parse(): passes NULL to parse_option_bool(), which treats it
  like "on".  Wrong if the value is actually false.  Bug can bite when
  qemu_opts_validate() runs after qemu_opt_set_bool().

* qemu_opt_del(): passes NULL to g_free(), which is just fine.

* qemu_opt_foreach(): passes NULL to the callback, which is unlikely to
  be prepared for it.

* qemu_opts_print(): prints NULL, which crashes on some systems.

* qemu_opts_to_qdict(): passes NULL to qstring_from_str(), which
  crashes.

It also makes qemu_opt_set_bool more readable by using find_desc_by_name
and opts_accepts_any.

It is based on Luiz's patch and uses Markus's comments. Discussions can
be found at:
http://lists.nongnu.org/archive/html/qemu-devel/2012-07/msg02716.html

Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
