hw/scsi: Fix debug message of cdb structure in scsi-generic

When running with debug enabled, the scsi-generic cdb that is
dumped skips byte 0 of the command, which is the opcode.  This
makes identifying which command is being issued/completed a
little difficult.  Example:

  0x00 0x00 0x01 0x00 0x00
  scsi-generic: scsi_read_data 0x0
  scsi-generic: Data ready tag=0x0 len=164
  scsi-generic: scsi_read_data 0x0
  scsi-generic: Command complete 0x0x10a42c60 tag=0x0 status=0

Improve this by adding a message prior to the loop, similar to
what exists for scsi-disk.  Clean up a few other messages to be
more explicit of what is being represented.  Example:

  scsi-generic: Command: data=0x12 0x00 0x00 0x01 0x00 0x00
  scsi-generic: scsi_read_data tag=0x0
  scsi-generic: Data ready tag=0x0 len=164
  scsi-generic: scsi_read_data tag=0x0
  scsi-generic: Command complete 0x0x10a452d0 tag=0x0 status=0

Signed-off-by: Eric Farman <farman@linux.vnet.ibm.com>
Message-Id: <20170120162527.66075-2-farman@linux.vnet.ibm.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
