update-linux-headers: prepare for hyperv.h removal

update-linux-headers: prepare for hyperv.h removal

All definitions related to Hyper-V emulation are now taken from the QEMU
own header, so the one imported from the kernel is no longer needed.

Unfortunately it's included by kvm_para.h.

So, until this is fixed in the kernel, teach the header harvesting
script to substitute kernel's hyperv.h with a dummy.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Message-Id: <20170713201522.13765-3-rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>