Fix bugs with read() and cfgstring handling

Don't use strtok, it changes the buffer.

Enlarge temp buffer in add_device.

Make sure read()s are null-terminated and cut off the trailing carriage
return too.

Signed-off-by: Andy Grover <agrover@redhat.com>
dummy: Implement using cfgstring for name of backing file

Also, convert to goto-based error handling.

Signed-off-by: Andy Grover <agrover@redhat.com>
dummy: Implement WRITE 10 support

Factor out common bits above the if/else.

Signed-off-by: Andy Grover <agrover@redhat.com>