selftests: KVM: Provide descriptive assertions in kvm_binary_stats_test

selftests: KVM: Provide descriptive assertions in kvm_binary_stats_test

As it turns out, tests sometimes fail. When that is the case, packing
the test assertion with as much relevant information helps track down
the problem more quickly.

Sharpen up the stat descriptor assertions in kvm_binary_stats_test to
more precisely describe the reason for the test assertion and which
stat is to blame.

Signed-off-by: Oliver Upton <oupton@google.com>
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Message-Id: <20220719143134.3246798-3-oliver.upton@linux.dev>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>