Rework hvm_wait_io() -- now tries to be a polite user of

Rework hvm_wait_io() -- now tries to be a polite user of
event channels by re-setting the selector and master
pending flags when it exits. Should make for better
behaviour when there are others using the event channels.

This needs some testing to be sure it doesn't break
anything or trigger latent bugs.

Signed-off-by: Keir Fraser <keir@xensource.com>