The Hyper-V ISR needs to live in a trampoline. Initially I tried
allocating a separate trampoline for it but it got somewhat complicated,
mainly due to the fact that the KENTER macro does not work in this
situation. In particular, it seems that LOAD_CR3's implementation
depends on having a fixed offset relative to tramp_idleptd. Another
detail 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.