Add virt-viewer download instructions for Windows

[qmfrederik]

What this PR does / why we need it:
Add virt-viewer download instructions for Windows. Took me some searching to figure this out, and thought it was worth documenting.

Which issue(s) this PR fixes

Special notes for your reviewer:

Release note:

NONE

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[rmohr]

@qmfrederik I guess that vnc access worked for you then without further modifications?

[qmfrederik]

@rmohr Yes it did!

[slintes]

great, but I'm afraid this won't work anymore after merge of #1673, because windows seems to be ignored there. Will request to add "remote-viewer" for windows over there.

[fabiand]

@slintes I see that #1673 did not get the windows support, correct?

[slintes]

@fabiand yes, tracking it in #1713

[fabiand]

Ack, then I think this one is good to go.