SCSI Command: only call unmarshall if we have an unmarshalling method

Some scsi commands do not read any datain from the device and thus
do not need an unmarshalling method.
Only invoke unmarshall iff the command has defined an unmarshalling method.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
TestUnitReady: add testunitready command

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Merge pull request #13 from sahlberg/testunitready

add Testunitready from sahlberg/python-scsi