s390x: Provide some more useful information if decryption of a PV image fails

JIRA: https://issues.redhat.com/browse/RHEL-18212

Provide a proper error message if the decryption of a "secure execution" image failed

Signed-off-by: Thomas Huth thuth@redhat.com

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Merge request included in qemu-kvm-8.2.0-3.el9