storage: add auth to virDomainDiskDef

storage: add auth to virDomainDiskDef

Add additional fields to let you specify the how to authenticate with a disk.
The secret to use may be referenced by a usage string or a UUID, i.e.:

<auth username='myuser'>
 <secret type='ceph' usage='secretname'/>
</auth>

or

<auth username='myuser'>
 <secret type='ceph' uuid='0a81f5b2-8403-7b23-c8d6-21ccc2f80d6f'/>
</auth>

Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>