Description of problem:

Creating a template from vm when source disk is on block storage, and the
volume does not have a parent fails with:

engine.log:

VDSM host3 command HSMGetAllTasksStatusesVDS failed: value=low level Image copy failed: ('Destination volume 0195a9f2-51f6-49c8-8520-9c630cbdfd59 error: Command [\'/usr/bin/qemu-img\', \'measure\', \'--output\', \'json\', \'-O\', \'qcow2\', \'json:{"file": {"driver": "file", "filename": "/rhev/data-center/mnt/blockSD/feab3738-c158-4d48-8a41-b5a95c057a50/images/3e8214f8-7a70-4e6b-a2fe-84c7103cd8ae/2eda0099-4dbc-4602-a418-fbd1ccee3357"}, "driver": "qcow2"}\'] failed with rc=1 out=b\'\' err=b\'qemu-img: Could not open \\\'json:{"file": {"driver": "file", "filename": "/rhev/data-center/mnt/blockSD/feab3738-c158-4d48-8a41-b5a95c057a50/images/3e8214f8-7a70-4e6b-a2fe-84c7103cd8ae/2eda0099-4dbc-4602-a418-fbd1ccee3357"}, "driver": "qcow2"}\\\': \\\'file\\\' driver requires \\\'/rhev/data-center/mnt/blockSD/feab3738-c158-4d48-8a41-b5a95c057a50/images/3e8214f8-7a70-4e6b-a2fe-84c7103cd8ae/2eda0099-4dbc-4602-a418-fbd1ccee3357\\\' to be a regular file\\n\'',) abortedcode=261

vdsm.log:

2021-06-17 03:14:17,571+0300 INFO  (tasks/1) [storage.ThreadPool.WorkerThread] START task 62624777-82b1-4d25-8db7-1bffbf34f
f90 (cmd=<bound method Task.commit of <vdsm.storage.task.Task object at 0x7fa62aee1f60>>, args=None) (threadPool:146)
2021-06-17 03:14:17,925+0300 INFO  (tasks/1) [storage.Image] sdUUID=feab3738-c158-4d48-8a41-b5a95c057a50 vmUUID= srcImgUUID
=3e8214f8-7a70-4e6b-a2fe-84c7103cd8ae srcVolUUID=2eda0099-4dbc-4602-a418-fbd1ccee3357 dstImgUUID=77eec8ed-7dce-4250-8575-42
5d661e64e6 dstVolUUID=0195a9f2-51f6-49c8-8520-9c630cbdfd59 dstSdUUID=feab3738-c158-4d48-8a41-b5a95c057a50 volType=6 volForm
at=COW preallocate=SPARSE force=False postZero=False discard=True (image:635)
2021-06-17 03:14:17,926+0300 INFO  (tasks/1) [storage.VolumeManifest] Volume: preparing volume feab3738-c158-4d48-8a41-b5a9
5c057a50/2eda0099-4dbc-4602-a418-fbd1ccee3357 (volume:599)
2021-06-17 03:14:17,934+0300 INFO  (tasks/1) [storage.LVM] Activating lvs: vg=feab3738-c158-4d48-8a41-b5a95c057a50 lvs=['2e
da0099-4dbc-4602-a418-fbd1ccee3357'] (lvm:1755)
2021-06-17 03:14:18,298+0300 ERROR (tasks/1) [storage.Image] Unexpected error (image:729)
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/vdsm/storage/image.py", line 673, in copyCollapsed
    sdUUID, volParams, dstSdUUID, dstVolFormat)
  File "/usr/lib/python3.6/site-packages/vdsm/storage/image.py", line 850, in calculate_vol_alloc
    return self.estimate_qcow2_size(src_vol_params, dst_sd_id)
  File "/usr/lib/python3.6/site-packages/vdsm/storage/image.py", line 117, in estimate_qcow2_size
    output_format=qemuimg.FORMAT.QCOW2)
  File "/usr/lib/python3.6/site-packages/vdsm/storage/qemuimg.py", line 156, in measure
    out = _run_cmd(cmd)
  File "/usr/lib/python3.6/site-packages/vdsm/storage/qemuimg.py", line 578, in _run_cmd
    raise cmdutils.Error(cmd, rc, out, err)
vdsm.common.cmdutils.Error: Command ['/usr/bin/qemu-img', 'measure', '--output', 'json', '-O', 'qcow2', 'json:{"file": {"driver": "file", "filename": "/rhev/data-center/mnt/blockSD/feab3738-c158-4d48-8a41-b5a95c057a50/images/3e8214f8-7a70-4e6b-a2fe-84c7103cd8ae/2eda0099-4dbc-4602-a418-fbd1ccee3357"}, "driver": "qcow2"}'] failed with rc=1 out=b'' err=b'qemu-img: Could not open \'json:{"file": {"driver": "file", "filename": "/rhev/data-center/mnt/blockSD/feab3738-c158-4d48-8a41-b5a95c057a50/images/3e8214f8-7a70-4e6b-a2fe-84c7103cd8ae/2eda0099-4dbc-4602-a418-fbd1ccee3357"}, "driver": "qcow2"}\': \'file\' driver requires \'/rhev/data-center/mnt/blockSD/feab3738-c158-4d48-8a41-b5a95c057a50/images/3e8214f8-7a70-4e6b-a2fe-84c7103cd8ae/2eda0099-4dbc-4602-a418-fbd1ccee3357\' to be a regular file\n'

The issues is using:

    'json:{"file": {"driver": "file", "filename": "/path"}, "driver": "qcow2"}'

When /path is block storage. It should be:

    'json:{"file": {"driver": "host_device", "filename": "/path"}, "driver": "qcow2"}'

The root cause is not calling qemuimg.measure() without is_block=True.

This is a regression introduce in:
$ git describe 1718b8784e841405574c44abe2357997e3235723
v4.40.32-4-g1718b8784

Before this change, qemu-img measure was called with a path instead of json: uri,
and qemu-img detected the right driver.

Version-Release number of selected component (if applicable):
vdsm-4.40.70.4

How reproducible:
100$

Steps to Reproduce:
1. Create think vm on block storage (iSCSI/FC)
   The vm disk must not have a parent.
2. Create template from the VM using qcow2 format

The operation fails.