Attach encryption information to virDomainDiskDef.

Attach encryption information to virDomainDiskDef.

The XML allows <encryption format='unencrypted'/>, this implementation
canonicalizes the internal representation so that "disk->encryption" is
non-NULL iff encryption information is available.

A domain with partial encryption information can be defined,
completeness of the information is not verified.  The domain won't
start until the remaining information is added, of course.

* docs/formatdomain.html, docs/formatdomain.html.in: Document
  new encryption options for disks
* docs/schemas/domain.rng: Pull in storage encryption schema
  rules
* src/domain_conf.h, src/domain_conf.c: Wire up storage encryption
  XML parsing/formatting APIs