Introduce get_ioengine for external engines

Introduce get_ioengine for external engines

This makes life easier for plugins written in C++
since they do not need to deal with struct initilization issues
with the ioengine_ops symbol.

With g++ a non-static ioengine_ops in global scope like this:

struct ioengine_ops ioengine = {
        .name           = "null",
        .version        = FIO_IOOPS_VERSION,
        .queue          = fio_null_queue,
        .commit         = fio_null_commit,
};

Results in:

cpp_null2.cc: At global scope:
cpp_null2.cc:112:1: error: C99 designator ‘name’ outside aggregate initializer
cpp_null2.cc:112:1: sorry, unimplemented: non-trivial designated initializers not supported
cpp_null2.cc:112:1: sorry, unimplemented: non-trivial designated initializers not supported
cpp_null2.cc:112:1: sorry, unimplemented: non-trivial designated initializers not supported
cpp_null2.cc:112:1: sorry, unimplemented: non-trivial designated initializers not supported
$

Example get_iongine() symbol usage:

---8<---
extern "C" {
void get_ioengine(struct ioengine_ops **ioengine_ptr) {
        struct ioengine_ops *ioengine;
        *ioengine_ptr = (struct ioengine_ops *) malloc(sizeof(struct ioengine_ops));
        ioengine = *ioengine_ptr;

        strcpy(ioengine->name, "cpp_null");
        ioengine->version        = FIO_IOOPS_VERSION;
        ioengine->queue          = fio_null_queue;
        ioengine->commit         = fio_null_commit;
        ioengine->getevents      = fio_null_getevents;
        ioengine->event          = fio_null_event;
        ioengine->init           = fio_null_init;
        ioengine->cleanup        = fio_null_cleanup;
        ioengine->open_file      = fio_null_open;
        ioengine->flags          = FIO_DISKLESSIO;
}
}
--->8---

Signed-off-by: Daniel Gollub <d.gollub@telekom.de>
Moved get_ioengine_t typedef to ioengine.h.

Signed-off-by: Jens Axboe <axboe@fb.com>