docs/system/virtio-pmem.rst: Fix minor style issues

docs/system/virtio-pmem.rst: Fix minor style issues

The virtio-pmem documentation has some minor style issues we hadn't
noticed since we weren't rendering it in our docs:

 * Sphinx doesn't complain about overlong title-underlining the
   way it complains about too-short underlining, but it looks odd;
   make the underlines of section headers the right length

 * Indent of paragraphs makes them render as blockquotes;
   remove the indent so they just render as normal text

 * Leading 'o' isn't rst markup, so it just renders as a literal
   "o"; reformat as a subsection heading instead

 * "QEMU" in the document title and section headings are a bit
   odd and unnecessary since this is the QEMU manual; delete
   or rephrase them

 * There's no need to specify what QEMU version the device first
   appeared in.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Pankaj Gupta <pankaj.gupta@cloud.ionos.com>