Switch from select() to poll() in xenconsoled's IO loop

Switch from select() to poll() in xenconsoled's IO loop

In Linux select() typically supports up to 1024 file descriptors. This can be
a problem when user tries to boot up many guests. Switching to poll() has
minimum impact on existing code and has better scalibility.

pollfd array is dynamically allocated / reallocated. If the array fails to
expand, we just ignore the incoming fd.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>