KVM: x86: Use wbinvd_on_cpu() instead of an open-coded equivalent

Use wbinvd_on_cpu() to target a single CPU instead of open-coding an
equivalent, and drop KVM's wbinvd_ipi() now that all users have switched
to x86 library versions.

No functional change intended.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20250522233733.3176144-6-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>