xen/intr: fix overflow of Xen interrupt range

The comparison was wrong. Hopefully this never occurred in the wild,
but now ensure the error message will occur before damage is caused.
This appears non-exploitable as exploitation would require a guest to
force Domain 0 to allocate all event channels, which a guest shouldn't
be able to do.

Adjust the error message to better describe what has occurred.

Reviewed by: royger
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30743