storage: Add support for using inputvol for encryption

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>