doc: add guide about virt-v2v

[mihalicyn]

Add a small example of using virt-v2v to convert Windows VMs to LXD.

Closes #12154

[tomponline]

Thanks!

[mihalicyn]

Hm, it's interesting:

Run make doc-lint
doc/.sphinx/.markdownlint/doc-lint.sh
Failed!
.tmp/doc/howto/import_machines_to_instances.md:46: MD034 Bare URL used
.tmp/doc/howto/import_machines_to_instances.md:47: MD034 Bare URL used
.tmp/doc/howto/import_machines_to_instances.md:110: MD034 Bare URL used
.tmp/doc/howto/import_machines_to_instances.md:214: MD034 Bare URL used

A detailed description of the rules is available at https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md
make: *** [Makefile:150: doc-lint] Error 1

I haven't used Bare URLs instead [description](link) syntax was used.
And lines 110 and 214 are not from this PR.

[tomponline]

@ru-fu is our expert on doc linting 😀

[ru-fu]

For the linting errors:

The last two were there before but had exceptions defined in https://github.com/canonical/lxd/blob/main/doc/.sphinx/.markdownlint/exceptions.txt
The line numbers changed now, so they need to be updated.

I'm not sure about the first two - there isn't really a reason. The first one might be because virtio-win.iso could be mistaken as a URL. If that's the case, putting it in quotes (which we need anyway since it's not a word) should fix it. The second is strange - but let's fix the others first, maybe it'll resolve itself by the other changes. ;)

[ru-fu]

Looks like it works in the output (except for the backticks in the summary, see comment).

And the strange linting errors disappeared as well! 🎉
For the two that are left, please update the line numbers in https://github.com/canonical/lxd/blob/main/doc/.sphinx/.markdownlint/exceptions.txt

[mihalicyn]

Now documentation tests/checks are happy!

[tomponline]

Thanks both!