libxl: events: Deregister xenstore watch fd when not needed

libxl: events: Deregister xenstore watch fd when not needed

We want to have no fd events registered when we are idle.
In this patch, deal with the xenstore watch fd:

* Track the total number of active watches.
* When deregistering a watch, or when watch registration fails, check
  whether there are now no watches and if so deregister the fd.
* On libxl teardown, the watch fd should therefore be unregistered.
  assert that this is the case.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Tested-by: Ian Campbell <ian.campbell@citrix.com>
Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>