s390x/cpu_model: disallow unpack for --only-migratable

s390x/cpu_model: disallow unpack for --only-migratable

Secure execution (aka protected virtualization) guests cannot be
migrated at the moment. If the unpack facility is provided in the cpu
model, a guest may choose to transition to secure mode, making the
guest unmigratable at that point in time. If the machine was explicitly
started with --only-migratable, we would get a failure only when the
guest actually tries to transition; instead, explicitly disallow the
unpack facility if --only-migratable was specified to avoid late
surprises.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Message-Id: <20210125135332.181324-1-borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>