qapi: Fix incorrect "Not documented" claims in QMP documentation

qapi: Fix incorrect "Not documented" claims in QMP documentation

Some qapi doc comments have forgotten the ':' after the
@argument, like this:

# @filename         Filename for the new image file
# @size             Size of the virtual disk in bytes

The result is that these are parsed as part of the body
text and appear as a run-on line:
  filename Filename for the new image file size Size of the virtual disk in bytes"
followed by
  filename: string
    Not documented
  size: int
    Not documented

in the 'Members' section.

Correct the formatting.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200213175647.17628-9-peter.maydell@linaro.org>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>