KVM: selftests: Print out the actual Top-Down Slots count on failure

Print out the expected vs. actual count of the Top-Down Slots event on
failure in the Intel PMU counters test.  GUEST_ASSERT() only expands
constants/macros, i.e. only prints the value of the expected count, which
makes it difficult to debug and triage failures.

Link: https://lore.kernel.org/r/20250117234204.2600624-6-seanjc@google.com


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