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
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