With vhost, the USER_POLL flags isn't passed to ioeventfd__add_event,
the function returns early and doesn't add the new event to the
used_ioevents list. As a result ioeventfd__del_event doesn't remove the
KVM event or free the structure. Always add the event to the list.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>