storage: Allow inputvol to be encrypted

storage: Allow inputvol to be encrypted

https://bugzilla.redhat.com/show_bug.cgi?id=1613737

When processing the inputvol for encryption, we need to handle
the case where the inputvol is encrypted. This then allows for
the encrypted inputvol to be used either for an output encrypted
volume or an output volume of some XML provided type.

Add tests to show the various conversion options when either input
or output is encrypted. This includes when both are encrypted.

Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>