test: also pass format for backing base image

mirroring what commit 9177cc2eda did
for the plugin system already, with QEMU 6.1 this is now a hard
requirement

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

test/list_volumes_test.pm

@@ -514,6 +514,7 @@ plan tests => $plan + 1;
 		my @cmd = ( '/usr/bin/qemu-img', 'create', "$file", DEFAULT_SIZE );
 		push @cmd, ( '-f', $suffix ) if $suffix;
 		push @cmd, ( '-u', '-b', @$parent[$num] ) if $parent;
+		push @cmd, ( '-F', $suffix ) if $parent && $suffix;
 		$num++;
 
 		run_command([@cmd]);