librbd: check for out of bounds I/O in all forms of read/write

librbd: check for out of bounds I/O in all forms of read/write

This was found by qemu-io tests, which tried to read and write past
the end of an rbd image. The test hung waiting for a completion that
was never scheduled, since it did not check the return value of
rbd_aio_write.

Reported-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>