scsi: add support for unit attention conditions

Unit attention conditions override any sense data the device already
has.  Their signaling and clearing is handled entirely by the SCSIBus
code, and they are completely transparent to the SCSIDevices.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
