[libvirt] Volume encryption missing pieces

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 .