spapr-vscsi: add task management

spapr-vscsi: add task management

At the moment the guest kernel issues two types of task management
requests to the hypervisor - task about and lun reset. This adds
handling for these tasks. As spapr-vscsi starts calling scsi_req_cancel(),
free_request callback was implemented.

As virtio-vscsi, spapr-vscsi does not handle CLEAR_ACA either as CDB
control byte does not seem to be used at all so NACA bit is not
set to the guest so the guest has no good reason to call CLEAR_ACA task.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[Fix choice of UCSOLCNT vs. SCSOLCNT. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>