oxenstored: add a poll-based select mechanism

oxenstored: add a poll-based select mechanism

Currently, oxenstored uses Unix.select underneath, so it doesn't work properly
if given a FD number >= 1024. This is a scalability bottleneck for hosts
running large number of VMs.

To remove this limitation, we implemented a poll-based mechanism but with the
same type signature as the Unix.select currently in use. So these two functions
can be interchangeable at any stage.

Signed-off-by: Zheng Li <dev@zheng.li>
Reviewed-by: David Scott <dave.scott@citrix.com>