Improve printf if dlopen fails

dlerror does what we want.

Signed-off-by: Andy Grover <agrover@redhat.com>
file_example: Remove unneeded include

Handlers just need tcmu-runner.h.

Signed-off-by: Andy Grover <agrover@redhat.com>
Clean up tcmu-runner.h

Work on a qemu handler highlighted some things we could improve in
tcmu-runner.h. We need to define struct iovec, declare handler_init, and
the name parameter to tcmu_get_attributes can be const.

Signed-off-by: Andy Grover <agrover@redhat.com>
main: Remove a debugprint

This stuff is working now so it's just noise.

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