virStorageFileGetMetadataRecurse: Allow format probing under special circumstances

virStorageFileGetMetadataRecurse: Allow format probing under special circumstances

Allow format probing to work around lazy clients which did not specify
their format in the overlay. Format probing will be allowed only, if we
are able to probe the image, the probing result was successful and the
probed image does not have any backing or data file.

This relaxes the restrictions which were imposed in commit 3615e8b39bad
in cases when we know that the image probing will not result in security
issues or data corruption.

We perform the image format detection and in the case that we were able
to probe the format and the format does not specify a backing store (or
doesn't support backing store) we can use this format.

With pre-blockdev configurations this will restore the previous
behaviour for the images mentioned above as qemu would probe the format
anyways. It also improves error reporting compared to the old state as
we now report that the backing chain will be broken in case when there
is a backing file.

In blockdev configurations this ensures that libvirt will not cause data
corruption by ending the chain prematurely without notifying the user,
but still allows the old semantics when the users forgot to specify the
format.

Users thus don't have to re-invent when image format detection is safe
to do.

The price for this is that libvirt will need to keep the image format
detector still current and working or replace it by invocation of
qemu-img.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>