poll: handle case of negative fds

poll_many() does not handle the case of negative fds.
Handle this case by setting revents = 0 and processing only
the fds which are >= 0.

Fixes #323

Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>