docs/specs/vmw_pvscsi-spec: Convert to rST

Convert the docs/specs/vmw_pvscsi-spec.txt file to rST format.
This conversion includes some minor wordsmithing of the text
to fix some grammar nits.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20230927151205.70930-2-peter.maydell@linaro.org