selftests: KVM: Add exponent check for boolean stats

The only sensible exponent for a boolean stat is 0. Add a test assertion
requiring all boolean statistics to have an exponent of 0.

Signed-off-by: Oliver Upton <oupton@google.com>
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Message-Id: <20220719143134.3246798-4-oliver.upton@linux.dev>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>