xen/intr: add reference counts to event channels

Add a reference count to xenisrc. This is required for implementation of
unmap-notifications in the grant table userspace device (gntdev). We need to
hold a reference to the event channel port, in case the user deallocates the
port before we send the notification.

Submitted by: jaggi
Reviewed by: royger
Differential review: https://reviews.freebsd.org/D7429