Remove the "scsi-block:" prefix for error messages as suggested
by Markus.

Improve the previous patch by making the message the same for both
scsi-block and scsi-generic, including the strerror() output in both
and making an explicit reference to SG_IO.  Also s/can not/cannot/.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>