hyperv: Fix vmbus after the i386 4/4 split

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