target/s390x/kvm/pv: Provide some more useful information if decryption fails

target/s390x/kvm/pv: Provide some more useful information if decryption fails

It's a common scenario to copy guest images from one host to another
to run the guest on the other machine. This (of course) does not work
with "secure execution" guests since they are encrypted with one certain
host key. However, if you still (accidentally) do it, you only get a
very user-unfriendly error message that looks like this:

 qemu-system-s390x: KVM PV command 2 (KVM_PV_SET_SEC_PARMS) failed:
  header rc 108 rrc 5 IOCTL rc: -22

Let's provide at least a somewhat nicer hint to the users so that they
are able to figure out what might have gone wrong.

Buglink: https://issues.redhat.com/browse/RHEL-18212
Message-ID: <20240110142916.850605-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>