scsi-disk: add need_fua_emulation to SCSIDiskClass

scsi-block will be able to do FUA just by passing the request through
to the LUN (which is also more efficient); there is no need to emulate
it like we do for scsi-disk.

Add a new method to distinguish this.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>