custom parameter handling fixes

custom parameter handling fixes

The recent changes to their handling introduced a few false warnings,
due to checks looking at the wrong string slot. While going through all
those commits and looking for patterns similar to the "dom0_mem=" I've
noticed this with, I also realized that there were other issues with
"dom0_nodes=" and "rmrr=", partly pre-existing, but partly also due to
those recent changes not having gone far enough.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>