Commit bab2eda6 changed the behavior for missing compat attribute,
but failed to update the documentation.

Before, the option was omitted from qemu-img command line and the
qemu-img default was used. Now we always specify the compat value
and the default is 0.10.

Reported by Christophe Fergeau
https://bugzilla.gnome.org/show_bug.cgi?id=746660#c4

(cherry picked from commit 7c8ae42d)
Signed-off-by: NJán Tomko <jtomko@redhat.com>