qapi/parser: preserve indentation in QAPIDoc sections

qapi/parser: preserve indentation in QAPIDoc sections

Change get_doc_indented() to preserve indentation on all subsequent text
lines, and create a compatibility dedent() function for qapidoc.py that
removes indentation the same way get_doc_indented() did.

This is being done for the benefit of a new qapidoc generator which
requires that indentation in argument and features sections are
preserved.

Prior to this patch, a section like this:

```
@name: lorem ipsum
   dolor sit amet
     consectetur adipiscing elit
```

would have its body text be parsed into:

```
lorem ipsum
dolor sit amet
  consectetur adipiscing elit
```

We want to preserve the indentation for even the first body line so that
the entire block can be parsed directly as rST. This patch would now
parse that segment into:

```
lorem ipsum
   dolor sit amet
     consectetur adipiscing elit
```

This is helpful for formatting arguments and features as field lists in
rST, where the new generator will format this information as:

```
:arg type name: lorem ipsum
   dolor sit amet
     consectetur apidiscing elit
```

...and can be formed by the simple concatenation of the field list
construct and the body text. The indents help preserve the continuation
of a block-level element, and further allow the use of additional rST
block-level constructs such as code blocks, lists, and other such
markup.

This understandably breaks the existing qapidoc.py; so a new function is
added there to dedent the text for compatibility. Once the new generator
is merged, this function will not be needed any longer and can be
dropped.

I verified this patch changes absolutely nothing by comparing the
md5sums of the QMP ref html pages both before and after the change, so
it's certified inert. QAPI test output has been updated to reflect the
new strategy of preserving indents for rST.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240626222128.406106-6-jsnow@redhat.com>
[Lost commit message paragraph restored]
Signed-off-by: Markus Armbruster <armbru@redhat.com>