virStorageEncryptionSecretCopy: Properly copy internals

virStorageEncryptionSecretCopy: Properly copy internals

virStorageEncryptionSecretPtr may have a string inside it, thus we must
copy the string too. Use virSecretLookupDefCopy to do that.

Caused by non-obvious code introduced in 756b46ddd24 and later 47e88b33b
which added a string that needed to be copied.

https://bugzilla.redhat.com/show_bug.cgi?id=1814923

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>