Bug#951356: virtual disk was allocated with -f qcow

[Chris Ward]

I have just noticed that the virtual disk for the system was allocated with
qemu-img create -f qcow Debian10.qcow 200G

where the intended allocation would be with

qemu-img create -f qcow2 Debian10.qcow2 200G

When I allocated the disk in the intended way, formatting proceeded much 
faster. This bug can be closed as 'user error'.