qemu: Add secinfo for hotplug virtio disk

qemu: Add secinfo for hotplug virtio disk

Commit id 'a1344f70a' added AES secret processing for RBD when starting
up a guest. As such, when the hotplug code calls qemuDomainSecretDiskPrepare
an AES secret could be added to the disk about to be hotplugged. If an AES
secret was added, then the hotplug code would need to generate the secret
object because qemuBuildDriveStr would add the "password-secret=" to the
returned 'driveStr' rather than the base64 encoded password.

Signed-off-by: John Ferlan <jferlan@redhat.com>