[handle] validate handle in public API

• add _is_valid_handle() function to verify if a handle is known, via qb_list*

• change all (and only) public API to use _is_valid_handle().

• move library config bits to lib_config.c and keep header bit in internal.h. this is more of a commodity need to simplify accessability of above function to the test suite at link time.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>