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

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: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>