Also make the inverse function, hyperv_find_vcpu, static as it's not
used outside hyperv.c

This paves the way to making hyperv.c built optionally.
Signed-off-by: NRoman Kagan <rkagan@virtuozzo.com>
Message-Id: <20180921082041.29380-3-rkagan@virtuozzo.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>