xen/hvc: make sure console output is always emitted, with explicit polling

We never want to rely on the hvc workqueue to emit output, because the
most interesting output is when the kernel is broken.  This will
improve oops/crash/console message for better debugging.

Instead, we force-poll until all output is emitted.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stable Kernel <stable@kernel.org>