mtx: print a usage message when no device is specified

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
mtx: print usage if there was no valid command specified

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
mtx: make sure return status is non-zero if the device was not a changer

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
mtx: fix how we calculate element addresses in the ui

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
libiscsi: add a module to access devices via libiscsi

Add a module to use libiscsi for device access.
Device names starting with /dev/sg will use the SG_IO module as before
but device names starting with iscsi:// will use the new libiscsi module.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Merge pull request #42 from sahlberg/iscsi

adding Iscsi support