x86: silence false log messages for plain "xpti" / "pv-l1tf"

x86: silence false log messages for plain "xpti" / "pv-l1tf"

While commit 2a3b34ec47 ("x86/spec-ctrl: Yet more fixes for xpti=
parsing")  claimed to have got rid of the 'parameter "xpti" has invalid
value "", rc=-22!' log message for "xpti" alone on the command line,
this wasn't the case (the option took effect nevertheless).

Fix this there as well as for plain "pv-l1tf".

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>