There already exists a virtio_blk_handle_write trace event as well as
completion events.  Add the virtio_blk_handle_read event so it's easy to
trace virtio-blk requests for both read and write operations.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>