qapi/migration.json: Replace _this_ with *this*

qapi/migration.json: Replace _this_ with *this*

The MigrationInfo::setup-time documentation is the only place where we
use _this_ inline markup for emphasis, commonly rendered in italics.
We would like to switch the doc comments to rST format, but rST
doesn't recognize that markup and emits literal underscores.

Switch to *this* instead.  Changes markup to strong emphasis with
Texinfo, commonly rendered as bold.  With rST, it will go right back
to emphasis / italics.

rST also uses **this** for strong (commonly rendered bold) where
Texinfo uses *this*. We have one place in the doc comments
which uses strong/bold markup, in qapi/introspect.json:
    Note: the QAPI schema is also used to help define *internal*

When we switch to rST that will be rendered as emphasis / italics.
Markus (who wrote that) thinks that using emphasis / italics
there is an improvement, so we leave that markup alone.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200213175647.17628-18-peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>