Rather than having storageBackendCreateQemuImgCheckEncryption
perform the virStorageGenerateQcowEncryption, let's just do that
earlier during storageBackendCreateQemuImg so that the check
helper is just a check helper rather doing something different
based on whether the format is qcow[2] or raw based encryption.

This fixes an issue in the storageBackendResizeQemuImg processing
for qcow encryption where if a secret was not available for a
volume, a new secret will not be generated and instead an error
message will be generated.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>