virtcontainers: Fix missing contexts in s390x

#1389 has added a context for many signatures to improve trace spans.
Functions specific to s390x lack this. Add context where required. This
affects some common code signatures, since some functions that do not
require context on other architectures do require it on s390x.
Also remove an unnecessary import in test_qemu_s390x.go.

Fixes: #1562

Signed-off-by: Jakob Naucke <jakob.naucke@ibm.com>

src/runtime/virtcontainers/qemu_arm64_test.go

@@ -6,6 +6,7 @@
 package virtcontainers
 
 import (
+	"context"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -121,7 +122,7 @@ func TestQemuArm64AppendImage(t *testing.T) {
 		},
 	}
 
-	devices, err = arm64.appendImage(devices, f.Name())
+	devices, err = arm64.appendImage(context.Background(), devices, f.Name())
 	assert.NoError(err)
 	assert.Equal(expectedOut, devices)