schemas: Add vim modeline

The various schemas included in QEMU use a JSON-based format which
is, however, strictly speaking not valid JSON.

As a consequence, when vim tries to apply syntax highlight rules
for JSON (as guessed from the file name), the result is an unreadable
mess which mostly consist of red markers pointing out supposed errors
in, well, pretty much everything.

Using Python syntax highlighting produces much better results, and
in fact these files already start with specially-formatted comments
that instruct Emacs to process them as if they were Python files.

This commit adds the equivalent special comments for vim.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Message-Id: <20200729185024.121766-1-abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>