engines/libblkio: Share a single blkio instance among threads in same process

engines/libblkio: Share a single blkio instance among threads in same process

fio groups all subjobs that set option 'thread' into a single process.
Have them all share a single `struct blkio` instance, with one `struct
blkioq` per thread/subjob. This allows benchmarking multi-queue setups.

Note that `struct blkio` instances cannot be shared across different
processes.

Signed-off-by: Alberto Faria <afaria@redhat.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>