tests: qemumonitorjson: Add test case for node name detection with blockjob

tests: qemumonitorjson: Add test case for node name detection with blockjob

qemu 2.9 returns an extra layer in the backing data if a block job is
active. Add a test case to see whether our code properly detects and
ignores such layer.

The test data was prepared by creating a backing chain of qcow2 images
(with qemu-img and with libvirt's snapshot feature).

One of the layers was then merged back by doing a block-commit:

virsh blockcommit VM hda --top /var/lib/libvirt/images/b

and then a block-copy job was started and kept in synchronized phase:

virsh blockcopy VM hda /tmp/tgt.img --transient job

Reviewed-by: Eric Blake <eblake@redhat.com>