sphinx: add qmp_lexer

sphinx: add qmp_lexer

Sphinx, through Pygments, does not like annotated json examples very
much. In some versions of Sphinx (1.7), it will render the non-json
portions of code blocks in red, but in newer versions (2.0) it will
throw an exception and not highlight the block at all. Though we can
suppress this warning, it doesn't bring back highlighting on non-strict
json blocks.

We can alleviate this by creating a custom lexer for QMP examples that
allows us to properly highlight these examples in a robust way, keeping
our directionality and elision notations.

Signed-off-by: Eduardo Habkost <[email protected]>
Signed-off-by: John Snow <[email protected]>
Reported-by: Aarushi Mehta <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Message-id: 20190603214653[email protected]
Signed-off-by: John Snow <[email protected]>