readcapacity16: add support for readcapacity16

Att scsi_cdb_readcapacity16.py. Add unmarshalling helpers for 16/32/64 bit
integers and provide a test_readcapacity16 test to demonstrate the api

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Merge pull request #7 from sahlberg/readcapacity16

readcapacity16: add support for readcapacity16