Don't require encryption password for 'qemu-img info' command

The encryption password is only required if I/O is going to be
performed on a disk image. The 'qemu-img info' command merely
reports metadata, so it should not ask for a decryption password

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
