xen/arm: vpl011: Add a new handle_console_ring function in xenconsole

xen/arm: vpl011: Add a new handle_console_ring function in xenconsole

This patch introduces a new handle_console_ring function. This function
reads the data from the ring buffer on receiving an event.

The initialization of event channel poll fd to -1 is moved inside the
handle_console_ring function as they are related. There should be no
change in the behavior as there is no functional change.

Signed-off-by: Bhupinder Thakur <bhupinder.thakur@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Wei Liu <wei.liu2@citrix.com>