evtchn: rename and adjust guest_enabled_event()

evtchn: rename and adjust guest_enabled_event()

The function isn't about an "event" in general, but about a vIRQ. The
function also failed to honor global vIRQ-s, instead assuming the caller
would pass vCPU 0 in such a case.

While at it also adjust the
- types the function uses,
- single user to make use of domain_vcpu().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>