qdev: Fix memory leak

The str allocated in visit_type_str was not freed.

The visit_type_str function is an input visitor(<QMP/String/etc>-to-native)
here, it will allocate memory for caller, so the caller is responsible for
freeing the memory.

Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: dunrong huang <riegamaths@gmail.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
