Several error messages print out the raw enum value, which
is less than helpful to users, as these values are not
documented, nor stable across QEMU releases. Switch to use
the enum string instead.

The nettle impl also had two typos where it mistakenly
said "algorithm" instead of "mode", and actually reported
the algorithm value too.
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>