qemu: fix nbdkit command test for backing chains

Previously this test only tested the generated nbdkit command for the
top level disk source. Update it to test the generated commmands for all
sources in the chain.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

tests/qemunbdkitdata/disk-network-ssh-key.args.disk0-src0

@@ -0,0 +1,9 @@
+SSH_AUTH_SOCK=/path/to/agent/socket \
+nbdkit \
+--unix /tmp/domain-QEMUGuest1/nbdkit-disk0-src0.socket \
+--foreground ssh \
+host=example.org \
+port=2222 \
+path=test1.img \
+user=myuser \
+known-hosts=/path/to/ssh_known_hosts1