block: extend BLOCK_IO_ERROR with reason string

BLOCK_IO_ERROR events are logged by libvirt, which helps with
post mortem analysis of guests. However, one information that
we miss today is a human readable string describing the cause
of the I/O error.

This commit adds that string it to BLOCK_IO_ERROR. Note that
this string is a debugging aid for humans, meaning that it
should not parsed by applications.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -3647,7 +3647,8 @@ static void send_qmp_error_event(BlockDriverState *bs,
     ac = is_read ? IO_OPERATION_TYPE_READ : IO_OPERATION_TYPE_WRITE;
     qapi_event_send_block_io_error(bdrv_get_device_name(bs), ac, action,
                                    bdrv_iostatus_is_enabled(bs),
-                                   error == ENOSPC, &error_abort);
+                                   error == ENOSPC, strerror(error),
+                                   &error_abort);
 }
 
 /* This is done by device models because, while the block layer knows

qapi/block-core.json

@@ -1593,6 +1593,10 @@
 #           io-status is present, please see query-block documentation
 #           for more information (since: 2.2)
 #
+# @reason: human readable string describing the error cause.
+#          (This field is a debugging aid for humans, it should not
+#           be parsed by applications) (since: 2.2)
+#
 # Note: If action is "stop", a STOP event will eventually follow the
 # BLOCK_IO_ERROR event
 #
@@ -1600,7 +1604,8 @@
 ##
 { 'event': 'BLOCK_IO_ERROR',
   'data': { 'device': 'str', 'operation': 'IoOperationType',
-            'action': 'BlockErrorAction', '*nospace': 'bool' } }
+            'action': 'BlockErrorAction', '*nospace': 'bool',
+            'reason': 'str' } }
 
 ##
 # @BLOCK_JOB_COMPLETED