KVM: selftests: Move 0/initial value PERF_CAPS checks to dedicated sub-test

Use a separate sub-test to verify userspace can clear PERF_CAPABILITIES
and restore it to the KVM-supported value, as the testcase isn't unique
to the LBR format.

Link: https://lore.kernel.org/r/20230311004618.920745-10-seanjc@google.com


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