test, librados: aio read *return_value consistency, fix ceph_test_rados_api_aio

test, librados: aio read *return_value consistency, fix ceph_test_rados_api_aio

test:
  Add set_completion*PP() functions to cast arg to correct class
  Add return_value checks
  Add some reads with buffers larger than object size
  Check buffer length on reads
librados:
  Make sure *return_value() has bytes read in all cases

Signed-off-by: David Zafman <david.zafman@inktank.com>