[libvirt] Volume encryption missing pieces

[Miloslav Trmač]

Major changes since the previous submission:
- Use one or two separate files per secret, using XML for metadata, instead
  of a single file for all secrets using a custom format.
- Split <secret> XML handling from the local driver into secret_conf.[ch],
  use it when auto-generating secrets.
- Use <usage type='volume'><volume/></usage> instead of <volume>.


Also available at http://fedorapeople.org/gitweb?p=mitr/public_git/libvirt.git;a=summary .