Define an <encryption> tag specifying volume encryption format and
format-depenedent parameters (e.g. passphrase, cipher name, key
length, key).

Currently the only defined parameter is a reference to a "secret"
(passphrase/key) managed using the virSecret* API.

Only the qcow/qcow2 encryption format, and a "default" format used to
let libvirt choose the format during volume creation, is currently
supported.

This patch does not add any users; the <encryption> tag is added in
the following patches to both volumes (to support encrypted volume
creation) and domains.

* docs/*.html: Re-generate
* docs/formatstorageencryption.html.in, docs/sitemap.html.in:
  Add page describing storage encryption data format
* docs/schemas/Makefile.am, docs/schemas/storageencryption.rng:
  Add RNG schema for storage encryption format
* po/POTFILES.in: Add src/storage_encryption_conf.c
* src/libvirt_private.syms: Export virStorageEncryption* functions
* src/storage_encryption_conf.h, src/storage_encryption_conf.c: Internal
  helper APIs for dealing with storage encryption format
* libvirt.spec.in, mingw32-libvirt.spec.in: Add storageencryption.rng
  RNG schema