tests: mock virRandomBits to make it endian stable
    
    virRandomBits is implemented in terms of virRandomBytes. Although we
    mock virRandomBytes to give a stable value, this is not sufficient to
    make virRandomBits give a stable value. The result of virRandomBits will
    vary depending on endianness. Thus we mock virRandomBits to return a
    stable value directly.
    
    Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
    Tested-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
    Reviewed-by: Eric Blake <eblake@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>