libxl: support multiple libxl__ev_fds for the same fd

libxl: support multiple libxl__ev_fds for the same fd

We need a slightly more sophisticated data structure to allow this,
where we record the slot not just for each fd but also for each
(fd,eventbit) where eventbit is POLLIN, POLLPRI, POLLOUT.

Document the new relaxed restriction.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Changes since v6:
 * Fix typo
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>