blktrace support: support for looking up a device

We can get the device number out of the trace, turn that into
a major/minor pair and look it up in /dev.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 6ee0d6a..2717f5d 100644
--- a/fio.h
+++ b/fio.h
@@ -268,6 +268,7 @@
 	 */
 	char *file_name;
 	void *mmap;
+	unsigned int major, minor;
 
 	/*
 	 * size of the file, offset into file, and io size from that offset