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

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

This patch introduces a new console_evtchn_unmask function. This function
unmasks the console event channel if it is masked for some timeout
period.

One optimization that has been done is to merge the two for loops.

One for loop was used to iterate through all domains and
unmask the domain event channels which had been rate limited for a
specified duration.

The other for loop was run to add the event channel fd and the tty fd to
the poll list.

These two for loops were merged so that the these operations can be done
in one iteration instead of two iterations.

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