Revert changes to sec label parsing

Revert changes to sec label parsing

Revert parsing changes:

  commit 302fe95ffa1bc5f1c61c0beb31a1adfbc38c668e
  Author: Eric Blake <eblake@redhat.com>
  Date:   Wed Jan 4 16:01:24 2012 -0700

    seclabel: fix regression in libvirtd restart

  commit b43432931aef92325920953ff92beabfbe5224c8
  Author: Eric Blake <eblake@redhat.com>
  Date:   Thu Dec 22 17:47:50 2011 -0700

    seclabel: allow a seclabel override on a disk src

These two commits changed the sec label parsing code so that
the same code dealt with both the VM level sec label, and the
per device label. Unfortunately, as we add more options to the
VM level sec label, the logic required to use the same parsing
code for the per device label becomes unintelligible.

* src/conf/domain_conf.c: Remove support for parsing per
  device sec labels