hyperv: Fix vmbus after the i386 4/4 split

The vmbus ISR needs to live in a trampoline. Dynamically allocating a
trampoline at driver initialization time poses some difficulties due to
the fact that the KENTER macro assumes that the offset relative to
tramp_idleptd is fixed at static link time. Another problem is that
native_lapic_ipi_alloc() uses setidt(), which assumes a fixed trampoline
offset.

Rather than fight this, move the Hyper-V ISR to i386/exception.s. Add a
new HYPERV kernel option to make this optional, and configure it by
default on i386. This is sufficient to make use of vmbus(4) after the
4/4 split. Note that vmbus cannot be loaded dynamically and both the
HYPERV option and device must be configured together. I think this is
not too onerous a requirement, since vmbus(4) was previously
non-functional.

Reported by: Harry Schmalzbauer <freebsd@omnilan.de>
Tested by: Harry Schmalzbauer <freebsd@omnilan.de>
Reviewed by: whu, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30577