The new functions are opts_accepts_any() and find_desc_by_name(), which
are also going to be used by qemu_opts_validate() (see next commit).

This also makes opt_set() slightly more readable.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NDong Xu Wang <wdongxu@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>