KVM: selftests: Add a helper to query if the PMU module param is enabled

Add a helper to probe KVM's "enable_pmu" param, open coding strings in
multiple places is just asking for false negatives and/or runtime errors
due to typos.

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20240109230250.424295-23-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>