x86: fix "xpti=" and "pv-l1tf=" yet again

x86: fix "xpti=" and "pv-l1tf=" yet again

While commit 2a3b34ec47 ("x86/spec-ctrl: Yet more fixes for xpti=
parsing") indeed fixed "xpti=dom0", it broke "xpti=no-dom0", in that
this then became equivalent to "xpti=no". In particular, the presence
of "xpti=" alone on the command line means nothing as to which default
is to be overridden; "xpti=no-dom0", for example, ought to have no
effect for DomU-s, as this is distinct from both "xpti=no-dom0,domu"
and "xpti=no-dom0,no-domu".

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