sheepdog: reorganize coroutine flow

sheepdog: reorganize coroutine flow

Delimit co_recv's lifetime clearly in aio_read_response.

Do a simple qemu_coroutine_enter in aio_read_response, letting
sd_co_writev call sd_write_done.

Handle nr_pending in the same way in sd_co_rw_vector,
sd_write_done and sd_co_flush_to_disk.

Remove sd_co_rw_vector's return value; just leave with no
pending requests.

[Jeff: added missing 'return' back, spotted by Paolo after
       series was applied.]

Signed-off-by: Jeff Cody <jcody@redhat.com>