qga/qapi-schema.json: minor format fixups for rST

qga/qapi-schema.json: minor format fixups for rST

We would like to switch the doc comments to rST format, and rST
requires a blank line before the start of a bulleted or enumerated
list. Two places in qapi-schema.json were missing this blank line.

Some places were using an indented line as a sort of single-item
bulleted list, which in the Texinfo output comes out all run
onto a single line; use a real bulleted list instead.

Some places unnecessarily indented lists, which confuses rST.

guest-fstrim:minimum's documentation was indented the
right amount to share a line with @minimum, but wasn't
actually doing so.

The indent on the bulleted list in the guest-set-vcpus
Returns section meant rST misindented it.

Changes to the generated Texinfo are very minor (the new
bulleted lists, and a few extra blank lines).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200213175647.17628-7-peter.maydell@linaro.org>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>