virCommandDoAsyncIO: Drop misleading statement about main event loop

virCommandDoAsyncIO: Drop misleading statement about main event loop

Back in v1.0.3-rc1~235 when I was adding virCommandDoAsyncIO(),
the main event loop was used to poll() on the pipe to the child
process. But this was promptly changed to a separate thread
handling I/O in v1.0.3-rc1~127. However, the corresponding
comment to virCommandDoAsyncIO() still documents the original
state.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>