qemu: Fix the command line generation for rbd auth using aes secrets

qemu: Fix the command line generation for rbd auth using aes secrets

https://bugzilla.redhat.com/show_bug.cgi?id=1182074

Since libvirt still uses a legacy qemu arg format to add a disk, the
manner in which the 'password-secret' argument is passed to qemu needs
to change to prepend a 'file.' If in the future, usage of the more
modern disk format, then the prepended 'file.' can be removed.

Fix based on Jim Fehlig <jfehlig@suse.com> posting and subsequent
upstream list followups, see:

http://www.redhat.com/archives/libvir-list/2016-August/msg00777.html

for details. Introduced by commit id 'a1344f70'.