libvchan: handle libxc evtchn failures properly in init functions

libvchan: handle libxc evtchn failures properly in init functions

The reasoning behind this patch is that ctrl->event_port is a uint32_t
(ie. unsigned), so the current checks on it for negative error results,
non-negative port presence etc. are incorrect.

Fix by using evtchn_port_or_error_t in the init functions instead,
adjusting the error handling, and removing the now-unnecessary check
from the close function.

Coverity-ID: 1055609
Coverity-ID: 1055610
Coverity-ID: 1055611
Signed-off-by: Matthew Daley <mattjd@gmail.com>
Reviewed-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>