No pretty printing in CXI sampling

[nscottnichols]

There is no pretty printing plugin for the CXI sampling backend. This is causing null termination characters for the interface names and counters to leak into the output on the CI nodes forcing us to use the -a flag with grep in the integration test:
integration_tests/sampling.bats

# assert there's at least one CXI counter sample in the trace
$THAPI_BIN_DIR/babeltrace_thapi --no-restrict cxi_trace_test \
  | grep -a "value: 9999"

Potential fixes are to either:

1. Add pretty print to CXI sampling backend
2. Add a deep strip function to babeltrace_thapi to strip the payload fields (str << (l ? l.call(e.payload_field.value) : e.payload_field.to_s.deep_strip))