qemu: block: Don't skip creation of 'luks' formatted images

libvirt treats 'luks' images as raw+encryption. The logic in
qemuBlockStorageSourceCreateFormat skipped the creation if the requested
image was raw but didn't take into account the encryption.

This manifested itself e.g. when attempting to do a virsh blockcopy with
the following XML:

    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/tmp/enccpy'>
        <encryption format='luks'>
          <secret type='passphrase' uuid='0a81f5b2-8403-7b23-c8d6-21ccc2f80d6f'/>
        </encryption>
      </source>
    </disk>

Where qemu would report the following error:

 unable to execute QEMU command 'blockdev-add': Volume is not in LUKS format

rather than actually formatting the image first.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

src/qemu/qemu_block.c

@@ -2479,7 +2479,8 @@ qemuBlockStorageSourceCreateFormat(virDomainObjPtr vm,
     g_autoptr(virJSONValue) createformatprops = NULL;
     int ret;
 
-    if (src->format == VIR_STORAGE_FILE_RAW)
+    if (src->format == VIR_STORAGE_FILE_RAW &&
+        !src->encryption)
         return 0;
 
     if (qemuBlockStorageSourceCreateGetFormatProps(src, backingStore,