KVM: x86: hyper-v: Always use to_hv_vcpu() accessor to get to 'struct kvm_vcpu_hv'

As a preparation to allocating Hyper-V context dynamically, make it clear
who's the user of the said context.

No functional change intended.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210126134816.1880136-11-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>