Fixes a regression introduced in
0b5e128750ed40d4348f2eb49d328b81dba9181a, whereby 'qemu-img create ...'
as used here would fail with:
qemu-img: disk.img: Backing file specified without backing format
thereby breaking tests from (gnu tests install).
* gnu/tests/install.scm (qemu-command/writable-image): Pass "-F qcow2".