When virCommandSetSendBuffer() is used over a virCommand that is
(or will be) daemonized, then the command must have
VIR_EXEC_ASYNC_IO flag set no later than at virCommandRunAsync()
phase so that the thread that's doing IO is spawned and thus
buffers can be sent to the process.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com> Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>