s390x/migration: Monitor commands for storage attributes

Add an "info" monitor command to non-destructively inspect the state of
the storage attributes of the guest, and a normal command to toggle
migration mode (useful for debugging).

Signed-off-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>