QA output created by 195
Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid

Change backing file of mid (opened read-only)

Testing: -drive if=none,file=TEST_DIR/t.IMGFMT,backing.node-name=mid
{
    QMP_VERSION
}
{
    "return": {
    }
}
{
    "return": {
    }
}
{
    "return": {
    }
}
{
    "timestamp": {
        "seconds":  TIMESTAMP,
        "microseconds":  TIMESTAMP
    },
    "event": "SHUTDOWN",
    "data": {
        "guest": false,
        "reason": "host-qmp-quit"
    }
}

image: TEST_DIR/t.IMGFMT.mid
file format: IMGFMT
virtual size: 64 MiB (67108864 bytes)
cluster_size: 65536
backing file: /dev/null
backing file format: IMGFMT

Change backing file of top (opened writable)

Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
Testing: -drive if=none,file=TEST_DIR/t.IMGFMT,node-name=top
{
    QMP_VERSION
}
{
    "return": {
    }
}
{
    "return": {
    }
}
{
    "return": {
    }
}
{
    "timestamp": {
        "seconds":  TIMESTAMP,
        "microseconds":  TIMESTAMP
    },
    "event": "SHUTDOWN",
    "data": {
        "guest": false,
        "reason": "host-qmp-quit"
    }
}

image: TEST_DIR/t.IMGFMT
file format: IMGFMT
virtual size: 64 MiB (67108864 bytes)
cluster_size: 65536
backing file: /dev/null
backing file format: IMGFMT
*** done