tests: Pass '-F qcow2' to 'qemu-img create'.

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".

1 files changed, 2 insertions, 2 deletions

diff --git a/gnu/tests/install.scm b/gnu/tests/install.scm
index 98de4c8359b..38785ae0bdb 100644
--- a/gnu/tests/install.scm
+++ b/gnu/tests/install.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2016, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2017, 2019, 2021 Tobias Geerinckx-Rice <me@tobias.gr>
 ;;; Copyright © 2020 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;; Copyright © 2020 Danny Milosavljevic <dannym@scratchpost.org>
@@ -355,7 +355,7 @@ IMAGE, a disk image.  The QEMU VM has access to MEMORY-SIZE MiB of RAM."
                 (format #t "creating writable image from '~a'...~%" image)
                 (unless (zero? (system* #+(file-append qemu-minimal
                                                        "/bin/qemu-img")
-                                        "create" "-f" "qcow2"
+                                        "create" "-f" "qcow2" "-F" "qcow2"
                                         "-o"
                                         (string-append "backing_file=" image)
                                         "disk.img"))