tools: rename libxc's evtchn_port_or_error_t with an xc_ prefix

tools: rename libxc's evtchn_port_or_error_t with an xc_ prefix

This is used only for xc_evtchn_alloc_unbound and the legacy/compat
versions of the old interfaces and avoids redefining the typedef. The
evtchn_port_or_error_t name is now used only be libxenevtchn.

None of the callers of xc_evtchn_alloc_unbound use the type
themselves.

NB xc_evtchn_alloc_unbound differs from xc_evtchn_bind_unbound_port
and the underlying xenevtchn_bind_unbound_port in that it allows the
specification of the local domain rather than assuming self. This is
only useful during domain build.

Reported-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>