doc: fix sphinx highlighting warnings

doc: fix sphinx highlighting warnings

Fix warnings raised by Python Sphinx 1.4.5:

    guides/sample_app_ug/ip_pipeline.rst:334:
    WARNING: Could not lex literal_block as "ini". Highlighting skipped.

    guides/sample_app_ug/l2_forward_real_virtual.rst:467:
    WARNING: Could not lex literal_block as "c". Highlighting skipped.

    guides/sample_app_ug/l3_forward.rst:293:
    WARNING: Could not lex literal_block as "c". Highlighting skipped.

    guides/sample_app_ug/vm_power_management.rst:162:
    WARNING: Could not lex literal_block as "xml". Highlighting skipped.

These warnings arise from invalid syntax in code-block directives.

Fixes: f1e779ec5b50 ("doc: update ip pipeline app guide")
Fixes: d0dff9ba445e ("doc: sample application user guide")
Fixes: c75f4e6a7a2b ("doc: add vm power mgmt app")

Signed-off-by: John McNamara <john.mcnamara@intel.com>