qemuBlockStorageSourceGetRBDProps: Simplify handling of encryption format
qemuBlockStorageSourceGetRBDProps: Simplify handling of encryption format

Remove the impossible error message about the 'qcow2' encryption format
not being supported. We validated before that it can't happen.

Additionally the code can be simplified by removing error handling from
impossible code paths as the last resort is virJSONValueCreate not
allowing NULL argument with the 's:' modifier.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

libvirt/libvirt,crobinso/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,olafhering/libvirt,crobinso/libvirt,jfehlig/libvirt,crobinso/libvirt,libvirt/libvirt,zippy2/libvirt,crobinso/libvirt,jfehlig/libvirt,olafhering/libvirt,jfehlig/libvirt,libvirt/libvirt,zippy2/libvirt