KVM: s390: Dynamic allocation of virtio-ccw I/O data.

Dynamically allocate any data structures like ccw used when
doing channel I/O. Otherwise, we'd need to add extra serialization
for the different callbacks using the same data structures.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>