docs/bitmaps: use QMP lexer instead of json

The annotated style json we use in QMP documentation is not strict json
and depending on the version of Sphinx (2.0+) or Pygments installed,
might cause the build to fail.

Use the new QMP lexer.

Further, some versions of Sphinx can not apply custom lexers to "code"
directives and require the use of "code-block" directives instead, so
make that change at this time as well.

Tested under:
- Sphinx 1.3.6 and Pygments 2.4
- Sphinx 1.7.6 and Pygments 2.2 (Fedora 29 packages)
- Sphinx 2.0.1 and Pygments 2.4
- Sphinx 3.0.0+/f396b3a783 and Pygments 2.4 (From Sphinx git c4f44bdd)

Reported-by: Aarushi Mehta <mehta.aaru20@gmail.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-id: 20190603214653.29369-4-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>