QA output created by 114
Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864 
Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base 
image: TEST_DIR/t.IMGFMT
file format: IMGFMT
virtual size: 64M (67108864 bytes)
cluster_size: 65536
backing file: TEST_DIR/t.IMGFMT.base
backing file format: foo
qemu-io: can't open device TEST_DIR/t.qcow2: Could not open backing file: Unknown driver 'foo'
read 4096/4096 bytes at offset 0
4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
*** done