Switch to poll() in cxenstored's IO loop

Switch to poll() in cxenstored's IO loop

Poll() can support more file descriptors than select(). We've done this for
xenconsoled, now do this for cxenstored as well.

The code is taken from xenconsoled and modified to adapt to cxenstored.

Note that poll() semantic is a bit different from select(). In Linux, if a fd
is set in IN/OUT fd_set and error occurs inside select(), this fd is still
considered readable / writable, and it is set in the returned IN/OUT fd_set.
So in later handle_input / handle_output, the connection will eventually be
talloc_free'ed(). After switching to poll(), we should take care of any error
right away, making the code clearer.

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