Add tests for backing chain handling, including a very long chain which
is fully specified in the XML and an unterminated chain.

The top level disk image would generate the following '-drive':

file-qcow2-backing-chain-encryption.xml:
-drive file=/var/lib/libvirt/images/a,encrypt.format=luks,
    encrypt.key-secret=node-b-f-encalias,format=qcow2,if=none,id=drive-dummy
-device virtio-blk-pci,scsi=off,drive=drive-dummy,id=dummy

file-qcow2-backing-chain-noopts.xml:
-drive file=/var/lib/libvirt/images/rhel7.3.1507297895,format=qcow2,if=none,id=drive-dummy
-device virtio-blk-pci,scsi=off,drive=drive-dummy,id=dummy

file-qcow2-backing-chain-unterminated.xml:
-drive file=/var/lib/libvirt/images/rhel7.3.1507297895,format=qcow2,if=none,id=drive-dummy
-device virtio-blk-pci,scsi=off,drive=drive-dummy,id=dummy
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>