KVM: selftests: Add checks for histogram stats bucket_size field

The bucket_size field should be non-zero for linear histogram stats and
should be zero for other stats types.

Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Jing Zhang <jingzhangos@google.com>
Message-Id: <20210802165633.1866976-4-jingzhangos@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>