qemu-options.hx: Fix formatting of -machine memory-backend option

qemu-options.hx: Fix formatting of -machine memory-backend option

The documentation of the -machine memory-backend has some minor
formatting errors:
 * Misindentation of the initial line meant that the whole option
   section is incorrectly indented in the HTML output compared to
   the other -machine options
 * The examples weren't indented, which meant that they were formatted
   as plain run-on text including outputting the "::" as text.
 * The a) b) list has no rst-format markup so it is rendered as
   a single run-on paragraph

Fix the formatting.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Igor Mammedov <[email protected]>
Message-id: 20210719105257[email protected]