libxenstore: Provide xs_check_watch

libxenstore: Provide xs_check_watch

Event-driven programs want to wait until the xs_fileno triggers for
reading, and then repeatedly call xs_check_watch.

Also xs_read_watch exposes a useless "num" out parameter, which should
always (if things aren't going hideously wrong) be at least 2 and
which the caller shouldn't be interested in.  So xs_check_watch
doesn't have one of those.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>