KVM: selftests: Use error codes to signal errors in PMU event filter test

Use '0' to signal success and '-errno' to signal failure in the PMU event
filter test so that the values are slightly less magical/arbitrary.  Using
'0' in the error paths is especially confusing as understanding it's an
error value requires following the breadcrumbs to the host code that
ultimately consumes the value.

Arguably there should also be a #define for "success", but 0/-errno is a
common enough pattern that defining another macro on top would likely do
more harm than good.

Link: https://lore.kernel.org/r/20230407233254.957013-5-seanjc@google.com


Reviewed by: Aaron Lewis <aaronlewis@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>