Instead of queuing messages when the control channels are full, xcs just

Instead of queuing messages when the control channels are full, xcs just
does nothing (see ctrl_interface.c:ctrl_chan_write_request()).

The following patch throttles the rate in which consoled writes data to
xcs.  With the current values, you get a responsive console but avoid
data corruption in most scenarios.

I'm able to get pretty far in my regression test with this patch.  With
higher throttle values I'm able to get even further (but the console
becomes painfully slow).

I implemented proper control channel queuing in xenctld in VM-Tools and
it's pretty nasty stuff.  This should prevent corruption for most users
until we can get rid of xcs.

Regards,

Anthony Liguori

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com