scsi-command.py: add functions to marshall 16/32/64 bit integers

Add functions scsi_16/32/64_to_ba that will masrshall integers
into a bytearray in big endian byte order.

Change the Inquiry function to use the 16 bit marshaller when writing
the allocation length to the CDB.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Merge pull request #4 from sahlberg/int-marshallers

scsi-command.py: add functions to marshall 16/32/64 bit integers