storage: Add support for using inputvol for encryption
    
    Starting with QEMU 2.9, encryption convert processing requires
    a multi-step process in order to generate an encrypted image from
    some non encrypted raw image.
    
    Processing requires to first create an encrypted image using the
    sizing parameters from the input source and second to use the
    --image-opts, -n, and --target-image-opts options along with inline
    driver options to describe the input and output files, generating
    two commands such as:
    
      $ qemu-img create -f luks \
          --object secret,id=demo.img_encrypt0,file=/path/to/secretFile \
          -o key-secret=demo.img_encrypt0 \
          demo.img 500K
      Formatting 'demo.img', fmt=luks size=512000 key-secret=demo.img_encrypt0
      $ qemu-img convert --image-opts -n --target-image-opts \
          --object secret,id=demo.img_encrypt0,file=/path/to/secretFile \
          driver=raw,file.filename=sparse.img \
          driver=luks,file.filename=demo.img,key-secret=demo.img_encrypt0
      $
    
    This patch handles the convert processing by running the processing
    in a do..while loop essentially reusing the existing create logic and
    arguments to create the target vol from the inputvol and then converting
    the inputvol using new arguments.
    
    This then allows the following virsh command to work properly:
    
      virsh vol-create-from default encrypt1-luks.xml data.img --inputpool default
    
    where encrypt1-luks.xml would provided the path and secret for
    the new image, while data.img would be the source image.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>
    ACKed-by: Michal Privoznik <mprivozn@redhat.com>