[Merge] livecd-rootfs:sil2100/appliance-qcow2 into livecd-rootfs:ubuntu/bionic

[Łukasz Zemczak]

The respective ubuntu-cdimage change for this:
https://code.launchpad.net/~sil2100/ubuntu-cdimage/qcow2-appliance/+merge/395210

Also, in all the other livecd-rootfs cases where qcow2 images are being generated, the .img extension is used instead of .qcow2. Here I decided on .qcow2 for two reasons:
1) Those are generated and exported alongside of the .img.xz images. I think if the qcow2 ones would be .img, people would confuse the two and think that these are the same as the .xz ones just uncompressed.
2) ubuntu-cdimage actually didn't have any handling of .img qcow2 images anyway, as these are only used for cloud images. So since I anyway had to be adding code to support this, why not use the actual extension?
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https://code.launchpad.net/~ubuntu-core-dev/livecd-rootfs/+git/livecd-rootfs/+merge/395209
Your team Ubuntu Core Development Team is requested to review the proposed merge of livecd-rootfs:sil2100/appliance-qcow2 into livecd-rootfs:ubuntu/bionic.