blkdebug: Add events and rules

Block drivers can trigger a blkdebug event whenever they reach a place where it
could be useful to inject an error for testing/debugging purposes.

Rules are read from a blkdebug config file and describe which action is taken
when an event is triggered. For now this is only injecting an error (with a few
options) or changing the state (which is an integer). Rules can be declared to
be active only in a specific state; this way later rules can distiguish on
which path we came to trigger their event.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block_int.h

@@ -120,6 +120,8 @@ struct BlockDriver {
     /* Returns number of errors in image, -errno for internal errors */
     int (*bdrv_check)(BlockDriverState* bs);
 
+    void (*bdrv_debug_event)(BlockDriverState *bs, BlkDebugEvent event);
+
     /* Set if newly created images are not guaranteed to contain only zeros */
     int no_zero_init;