Provide missing passphrase when creating a volume.

Provide missing passphrase when creating a volume.

If the <encryption format='qcow'> element does not specify a secret
during volume creation, generate a suitable secret and add it to the
<encryption> tag.  The caller can view the updated <encryption> tag
using virStorageVolGetXMLDesc().

Similarly, when <encryption format='default'/> is specified while
creating a qcow or qcow2-formatted volume, change the format to "qcow"
and generate a secret as described above.

* src/storage_encryption_conf.h (VIR_STORAGE_QCOW_PASSPHRASE_SIZE,
  virStorageGenerateQcowPasphrase),
  src/storage_encryption_conf.c (virStorageGenerateQcowPasphrase),
  src/libvirt_private.syms: Add virStorageGenerateQcowPasphrase().
* src/storage_backend.c (virStoragegenerateQcowEncryption,
  virStorageBackendCreateQemuImg): Generate a passphrase and
  <encryption> when creating a qcow-formatted encrypted volume and the
  user did not supply the information.