qmp: Documentation for BLOCK_IMAGE_CORRUPTED

Add an appropriate entry describing this event and its parameters into
qmp-events.txt.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>