KVM: Refactor and document halt-polling stats update helper

Add a comment to document that halt-polling is considered successful even
if the polling loop itself didn't detect a wake event, i.e. if a wake
event was detect in the final kvm_vcpu_check_block().  Invert the param
to update helper so that the helper is a dumb function that is "told"
whether or not polling was successful, as opposed to determining success
based on blocking behavior.

Opportunistically tweak the params to the update helper to reduce the
line length for the call site so that it fits on a single line, and so
that the prototype conforms to the more traditional kernel style.

No functional change intended.

Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>