Expose virtual size of backing images

[tserong]

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Which issue(s) this PR fixes:

https://github.com/longhorn/longhorn/issues/7923

What this PR does / why we need it:

This commit adds virtualSize to the BackingImageStatus and BackingImageFileInfo structs, and thus to the BackingImage and BackingImageManager CRDs. We can see how this works in practice by creating a new backing image downloaded from a URL, then periodically running kubectl -n longhorn-system get lhbi. I'm using https://download.opensuse.org/distribution/leap/15.5/appliances/openSUSE-Leap-15.5-Minimal-VM.x86_64-Cloud.qcow2 in the example below.

Initially, when the download is just started, before the file size is known:

> kubectl -n longhorn-system get lhbi/default-image-7mplj
NAME                  UUID       SOURCETYPE   SIZE        VIRTUALSIZE   AGE
default-image-7mplj   6d5a98b0   download     0           0             9s

A little later, while the download is running:

> kubectl -n longhorn-system get lhbi/default-image-7mplj
NAME                  UUID       SOURCETYPE   SIZE        VIRTUALSIZE   AGE
default-image-7mplj   6d5a98b0   download     255701504   0             3m33s

Finally, once the download is complete:

> kubectl -n longhorn-system get lhbi/default-image-7mplj
NAME                  UUID       SOURCETYPE   SIZE        VIRTUALSIZE   AGE
default-image-7mplj   6d5a98b0   download     255701504   821035008     4m26s

Compare size and virtualSize above with the image downloaded manually:

> wget https://download.opensuse.org/distribution/leap/15.5/appliances/openSUSE-Leap-15.5-Minimal-VM.x86_64-Cloud.qcow2
[...]

> ls -l openSUSE-Leap-15.5-Minimal-VM.x86_64-Cloud.qcow2
-rw-r--r-- 1 tserong users 255701504 Dec 19 23:26 openSUSE-Leap-15.5-Minimal-VM.x86_64-Cloud.qcow2

> qemu-img info openSUSE-Leap-15.5-Minimal-VM.x86_64-Cloud.qcow2 | grep virtual
virtual size: 783 MiB (821035008 bytes)

Special notes for your reviewer:

This PR manually pulls the changes from https://github.com/longhorn/backing-image-manager/pull/208 into the vendor directory, so probably shouldn't be taken as-is (I assume we need to merge the BIM PR first, then do a proper go get ; go mod tidy ; go mod vendor thing separately to pull the BIM changes into longhorn-manager, and only then look at this PR, but please correct me if I'm wrong).

Additional documentation or context

N/A