virsh: add snapshot backing store support to vol-create-as

virsh: add snapshot backing store support to vol-create-as

This patch adds two new parameters to the vol-create-as command:

 --backing-vol <volume-name-or-key-or-path>
 --backing-vol-format <format-of-backing-vol>

  virsh # vol-create-as guest_images_lvm snapvol1 5G --backing-vol \
              rhel6vm1lun1
  Vol snapvol1 created

  virsh # vol-create-as image_dir qcow2snap2 5G --format qcow2 \
              --backing-vol imagevol1.qcow2 \
              --backing-vol-format qcow2
  Vol qcow2snap2 created

Additionally, the virsh man page update fixes incorrect snapshot
parameters that were included in my prior bulk volume command patch.