crypto: extend unit tests to cover decryption too

The current unit test only verifies the encryption API,
resulting in us missing a recently introduced bug in the
decryption API from commit d3462e3. It was fortunately
later discovered & fixed by commit bd09594, thanks to the
QEMU I/O tests for qcow2 encryption, but we should really
detect this directly in the crypto unit tests. Also remove
an accidental debug message and simplify some asserts.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1437468902-23230-1-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
