Add support for QCOW images

Note that the disk.qcow2 is a QCOW version 3 file, which is very similar
to version 2. I haven't found the .qcow3 extension used in the wild
though.
1 job for master in 6 minutes and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #617134
build

00:06:19