hyperv/evttimer: Use an independent message slot so that it can work

Using the same message slot as the other types of the messages has
the side effect that the event timer message could be deferred to
the swi threads to run (lacking of trapframe and the original code
didn't even handle that, so the event timer was actually broken).

As of this commit we use an independent message slot for event timer,
so that we could handle all of event timer messages in the interrupt
handler directly. Note, the message slot for event timer is still
bind to the same interrupt vector as the other types of messages.

Submitted by: Jun Su <junsu microsoft com>
Reviewed by: sephe
Discussed with: Jun Su <junsu microsoft com>, Dexuan Cui <decui microsoft com>
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5696