qcow: fix memory leaks related to encryption

Fix leak of the 'encryptopts' string, which was mistakenly
declared const.

Fix leak of QemuOpts entry which should not have been deleted
from the opts array.

Reported by: coverity
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170714103105.5781-1-berrange@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
