vhost: allow many vhost-user ports

vhost: allow many vhost-user ports

Currently select() is used to monitor file descriptors for vhostuser
ports. This limits the number of ports possible to create since the
fd number is used as index in the fd_set and we have seen fds > 1023.
This patch changes select() to poll(). This way we can keep an
packed (pollfd) array for the fds, e.g. as many fds as the size of
the array.

Also see:
http://dpdk.org/ml/archives/dev/2016-April/037024.html

Reported-by: Patrik Andersson <patrik.r.andersson@ericsson.com>
Signed-off-by: Jan Wickbom <jan.wickbom@ericsson.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>