A significant part of hyperv.c is not actually tied to x86, and can
be moved to hw/.

This will allow to maintain most of Hyper-V and VMBus
target-independent, and to avoid conflicts with inclusion of
arch-specific headers down the road in VMBus implementation.

Also this stuff can now be opt-out with CONFIG_HYPERV.
Signed-off-by: NRoman Kagan <rkagan@virtuozzo.com>
Message-Id: <20180921082041.29380-4-rkagan@virtuozzo.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>